[media] em28xx: remove obsolete device state checks from the ioctl functions

v4l2_device_disconnect() is called when the device is disconnected, so that the
v4l2-core rejects all ioctl calls.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>