media: dw2102: return -EIO instead of -1 for mac address read errors

The dvb-usb core function only checks if the returned value is
zero, so it doesn't actually matter the error code. Still, returning
-1 as an error condition is something that we don't do upstream. So,
change the logic to return -EIO in case of I2C transfer failures.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>