[media] em28xx: do not interpret eeprom content if eeprom key is invalid

If the eeprom key isn't valid, either a different (currently unknown) format
is used or the eeprom is corrupted.
In both cases it doesn't make sense to interpret the data.
Also print an error message.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>