media: ov5640: Make the FPS clamping / rounding more extendable

The current code uses an algorithm to clamp the FPS values and round them
to the closest supported one that isn't really allows to be extended to
more than two values.

Rework it a bit to make it much easier to extend the amount of FPS options
we support.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Adam Ford <aford173@gmail.com> #imx6dq
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>