[media] rainshadow-cec: ensure exit_loop is intialized

exit_loop is not being initialized, so it contains garbage. Ensure it is
initialized to false.

Detected by CoverityScan, CID#1436409 ("Uninitialized scalar variable")

Fixes: ea6a69de ("[media] rainshadow-cec: avoid -Wmaybe-uninitialized warning")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>