media: ar0521: Refuse unsupported controls

Refuse unsupported controls by returning -EINVAL in the s_ctrl
operation. While at it, remove a the default switch case in the first
switch as it effectively is now a no-op.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>