media: ar0521: Remove redundant variable ret

ret in ar0521_set_fmt is never set to values other than 0. Replace it with
plain 0.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>