[media] em28xx: remove field tda9887_conf from struct em28xx

The tda9887 chipset is part of the analog tuner. Move it out of
em28xx-cards.

Also, it is used only one time by the v4l2 sub-module at tuner setup.

With that, we can get rid of an additional data inside the em28xx
common structure.

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>