[media] cx23885: map invalid fields to a valid field

If field format is not valid, map it as V4L2_FIELD_INTERLACED,
instead of pretending to support an invalid format.

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