media: ov5640: Make 2592x1944 mode only available at 15 fps

The sensor data sheet clearly state that 2592x1944 only works at 15 fps
make sure we don't try to miss configure the pll out of acceptable
range.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>