[media] em28xx: fix+improve+unify i2c error handling, debug messages and code comments

- do not pass USB specific error codes to userspace/i2c-subsystem
- unify the returned error codes and make them compliant with
  the i2c subsystem spec
- check number of actually transferred bytes (via USB) everywehere
- fix/improve debug messages
- improve code comments

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