Fix abrupt USB disconnection leaving stale data in DAP queue buffers.

Clearing the buffers in the USB_DISCONNECTING state, which then
moves to the USB_DISCONNECTED state.

The USB_DISCONNECTING state was never used, in the current codebase
nor the initial repo commit (well, second commit, first release):
bdacee7

It looks like this state might have been initialy designed as a way
to trigger a USB disconnection from the interface chip, as it
manually set usbd_connect(0) to disconnect.
As it was never used it's now the pre-disconnected state to run
code only once on state change:
connected->disconnecting->disconnected

Fixes ARMmbed#1089

Author: microbit-carlos

source/board/microbitv2/kl27z/power.c

@@ -106,7 +106,7 @@ void PORTCD_IRQHandler(void)
             USBD_Reset();
             usbd_reset_core();
             usb_pc_connected = false;
-            usb_state = USB_DISCONNECTED;
+            usb_state = USB_DISCONNECTING;
         }
         else {
             // Cable inserted

source/board/microbitv2/nrf52820/power.c

@@ -247,7 +247,7 @@ void GPIOTE_IRQHandler(void)
                 USBD_Reset();
                 usbd_reset_core();
                 usb_pc_connected = false;
-                usb_state = USB_DISCONNECTED;
+                usb_state = USB_DISCONNECTING;
             } else {
                 // Cable inserted
                 wake_from_usb = 1;
@@ -272,7 +272,7 @@ void POWER_CLOCK_IRQHandler(void)
         USBD_Reset();
         usbd_reset_core();
         usb_pc_connected = false;
-        usb_state = USB_DISCONNECTED;
+        usb_state = USB_DISCONNECTING;
     }
 }
 #endif // POWER_IRQ_USBDETECTED

source/daplink/interface/main_interface.c

@@ -401,9 +401,11 @@ void main_task(void * arg)
             switch (usb_state) {
                 case USB_DISCONNECTING:
                     usb_state = USB_DISCONNECTED;
-                    // Disable board power before USB is disconnected.
+                    // Disable target power as USB was disconnected.
                     gpio_set_board_power(false);
-                    usbd_connect(0);
+                    // Clear the DAP queue buffers
+                    usbd_bulk_init();
+                    usbd_hid_init();
                     break;
 
                 case USB_CONNECTING:
@@ -435,12 +437,16 @@ void main_task(void * arg)
                     break;
 
                 case USB_CONNECTED:
+                    if (!usbd_configured()) {
+                        usb_state = USB_DISCONNECTING;
+                    }
+                    break;
+
                 case USB_DISCONNECTED:
                     if (usbd_configured()) {
                         usb_state = USB_CONNECTED;
                     }
                     else {
-                        usb_state = USB_DISCONNECTED;
                         usb_state_count = USB_CONNECT_DELAY;
                         usb_no_config_count = USB_CONFIGURE_TIMEOUT;
                     }