media: staging: greybus: light: fix memory leak in v4l2 register

We are allocating memory for the v4l2 flash configuration structure and
leak it in the normal path. Just use the stack for this as we do not
use it outside of this function.

Also use IS_ERR() instead of IS_ERR_OR_NULL() to check return value from
v4l2_flash_init() for it never returns NULL.

Fixes: 2870b52b

 ("greybus: lights: add lights implementation")

Reported-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>