media: atmel-isi: Use accessors for pad config 'try_*' fields

The 'try_*' fields of the v4l2_subdev_pad_config structure are meant to
be accessed through helper functions. Replace direct access with usage
of the v4l2_subdev_get_pad_format(), v4l2_subdev_get_pad_crop() and
v4l2_subdev_get_pad_compose() helpers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>