[media] m5602_s5k83a: check return value of kthread_create

Function kthread_create() returns an ERR_PTR on error. However, in
function s5k83a_start(), its return value is used without validation.
This may result in a bad memory access bug. This patch fixes the bug.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>