[media] stk1160: fix sequence handling

Fix the sequence counter: we're counting frames, not fields.

Also remove the unused 'field' field. That would only be needed if this driver
would support V4L2_FIELD_ALTERNATE.

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