[media] mem2mem: replace BUG_ON with WARN_ON

See following thread for rationale:
	http://www.spinics.net/lists/linux-media/msg52462.html
Tested by compilation only.
Signed-off-by: NNicolas Thery <nicolas.thery@st.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/v4l2-core/v4l2-mem2mem.c

@@ -510,12 +510,10 @@ struct v4l2_m2m_dev *v4l2_m2m_init(struct v4l2_m2m_ops *m2m_ops)
 {
 	struct v4l2_m2m_dev *m2m_dev;
 
-	if (!m2m_ops)
+	if (!m2m_ops || WARN_ON(!m2m_ops->device_run) ||
+			WARN_ON(!m2m_ops->job_abort))
 		return ERR_PTR(-EINVAL);
 
-	BUG_ON(!m2m_ops->device_run);
-	BUG_ON(!m2m_ops->job_abort);
-
 	m2m_dev = kzalloc(sizeof *m2m_dev, GFP_KERNEL);
 	if (!m2m_dev)
 		return ERR_PTR(-ENOMEM);