[media] em28xx: fill in readbuffers and fix incorrect return code

g/s_parm should fill in readbuffers.
For non-webcams s_parm should return -ENOTTY instead of -EINVAL.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>