usb: usbnet: use newly introduced hex_to_bin()

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: linux-usb@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>