media: ov5640: Make the return rate type more explicit

In the ov5640_try_frame_interval function, the ret variable actually holds
the frame rate index to use, which is represented by the enum
ov5640_frame_rate in the driver.

Make it more obvious.

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>