media: common: saa7146: fix compliance problems with field handling

The initial field value of the format is ANY, which isn't allowed.
Change to INTERLACED.

VIDIOC_TRY_FMT will overwrite vv->last_field, which is an
unwanted side-effect, so drop this.

And finally vidioc_s_fmt_vid_cap didn't initialize vv->last_field
correctly.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>