[media] rainshadow-cec: Fix missing spin_lock_init()

The driver allocates the spinlock but not initialize it.
Use spin_lock_init() on it to initialize it correctly.

This is detected by Coccinelle semantic patch.

Fixes: 0f314f6c ("[media] rainshadow-cec: new RainShadow Tech HDMI
CEC driver")

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>