media: v4l2-flash: Check whether setting LED brightness succeeded

Setting LED brightness may return an error but the return value was never
checked by the V4L2 flash LED class. Do it now.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>