media: ov5640: Fix set 15fps regression

The ov5640_try_frame_interval operation updates the FPS as per user
input based on default ov5640_frame_rate, OV5640_30_FPS which is failed
to update when user trigger 15fps.

So, initialize the default ov5640_frame_rate to OV5640_15_FPS so-that
it can satisfy to update all fps.

Fixes: 5a3ad937 ("media: ov5640: Make the return rate type more explicit")

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-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>