media: ov5670: Limit vblank to permissible range

Previously, vblank range given to user was too big, falling outside
of permissible range for a given resolution. Sometimes, too low vblank
resulted in errors.

Now, limit vblank to only permissible range for a given resolution.
This change limits lower-bounds of vblank, doesn't affect upper bounds.

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