media: ov7740: Check for possible NULL return value in control creation

Check that creating the control actually succeeded before accessing it.
A failure would lead to NULL pointer reference. Fix this.

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