ethernet: mdio: Shell Usability: list devices on error

[clamattia]

Summary

• Other shells list all available devices on error.
• MDIO_SHELL: only reports the error device (null) not found

Describe the solution you'd like

On error, the shell shall print the available devices. Like for example SPI_SHELL:

spi conf
conf: wrong parameter count
conf - Configure SPI
       Usage: spi conf <device> <frequency> [<settings>]
       <settings> - any sequence of letters:
       o - SPI_MODE_CPOL
       h - SPI_MODE_CPHA
       l - SPI_TRANSFER_LSB
       T - SPI_FRAME_FORMAT_TI
       example: spi conf spi1 1000000 ol
Subcommands:
  spi@40003800

Alternatives

None

Additional Context

None