media: adv7180: fix field type to V4L2_FIELD_ALTERNATE

The ADV7180 and ADV7182 transmit whole fields, bottom field followed
by top (or vice-versa, depending on detected video standard). So
for chips that do not have support for explicitly setting the field
mode via I2P, set the field mode to V4L2_FIELD_ALTERNATE.

I2P converts fields into frames using an edge adaptive algorithm. The
frame rate is the same as the 'field rate': e.g. X fields per second
are now X frames per second.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>