[media] stk1160: Replace BUG_ON with WARN_ON

This situation is not even an error condition so it's stupid to BUG_ON.
Learn the lesson:
http://permalink.gmane.org/gmane.linux.kernel/1347333



Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>