media: ov7740: Fix control handler error at the end of control init

Check that no error happened during adding controls to the driver's
control handler. Print an error message and bail out if there was one.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>