media: imon: Fix race getting ictx->lock

Lets fix a race between mutex_is_lock() and mutex_lock().

<-mutex is not locked
if (!mutex_is_locked(&ictx->lock)) {
	unlock = true; <- mutex is locked externaly
	mutex_lock(&ictx->lock);
}

Let's use mutex_trylock() that does mutex_is_lock() and mutex_lock()
atomically.

Fix the following cocci warning:
drivers/media/rc/imon.c:1167:1-7: preceding lock on line 1153

Fixes: 23ef710e ("[media] imon: add conditional locking in change_protocol")
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>