media: ov5647: Implement set_fmt pad operation

Now that the driver supports more than a single mode, implement the
.set_fmt pad operation and adjust the existing .get_fmt one to report
the currently applied format.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>