media: imx274: fix frame interval handling

1) the numerator and/or denominator might be 0, in that case
   fall back to the default frame interval. This is per the spec
   and this caused a v4l2-compliance failure.

2) the updated frame interval wasn't returned in the s_frame_interval
   subdev op.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>