[media] cx23885: fix field handling

Add missing SEQ_BT/TB support, bottom field is first for all 60 Hz formats,
not just NTSC, restore an overwritten field value and initialize dev->field
correctly.

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