[media] em28xx: move capture state tracking fields from struct em28xx to struct v4l2

Move some temporary capture tracking date to the em28xx_v4l2 struct,
as those info are used only by em28xx v4l2 submodule.

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>