drivers: mdio: shell: Fix various buffer overflows

[clamattia]

Need to check argument count to avoid buffer overflows and crashes.

[clamattia]

Related: #93254

[sonarqubecloud]

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[tpambor]

I think this and the below new checks are not needed as the commands take no optional arguments and therefore this is already checked by the shell.

zephyr/include/zephyr/shell/shell.h

Lines 531 to 532 in 72dc0e5

	* @note If a command will be called with wrong number of arguments shell will
	* print an error message and command handler will not be called.

[tpambor]

I think the number of mandatory arguments for the scan command should be fixed, currently 1 but should be 2, scan and device.

	SHELL_CMD_ARG(scan, &dsub_device_name,
		"Scan MDIO bus for devices: scan <device> [<reg_addr>]",
		cmd_mdio_scan, 1, 1),