usb gadget ether: Fix warning in is_eth_addr_valid()

The gadget ethernet driver needs to keep copies of the MAC address (at
both endpoints) as strings so it needs a custom function for validation
of the MAC.  It was not however performing a totally correct check and
also was emitting a warning about a set but unused variable.  The
solution to both is that after checking the string contents we use the
standard test for a valid MAC.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>