When adding proper support for V4L2_FIELD_ALTERNATE it was missed that
this field format should trigger an interrupt for each field, not just
for the whole frame. Fix this by marking it as progressive in the
capture setup, which will then select the correct interrupt mode.

Tested on both Gen2 and Gen3 with the result of a doubling of the frame
rate for V4L2_FIELD_ALTERNATE. From a PAL video source the frame rate is
now 50, which is expected for alternate field capture.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>