[media] em28xx: move v4l2 user counting fields from struct em28xx to struct v4l2

Despite being at the common em28xx struct, those two fields are
actually taking into account only the usage inside em28xx v4l2
submodule. So, move them out of the common struct.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>