media: led-class-flash: better handle NULL flash struct

The logic at V4L2 led core assumes that the flash struct
can be null. However, it doesn't check for null while
trying to set, causing some smatch  to warn:

	drivers/media/v4l2-core/v4l2-flash-led-class.c:210 v4l2_flash_s_ctrl() error: we previously assumed 'fled_cdev' could be null (see line 200)

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