[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: Nicolas Thery <nicolas.thery@st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
index 3ac8358..017fed8 100644
--- a/drivers/media/v4l2-core/v4l2-mem2mem.c
+++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
@@ -510,12 +510,10 @@
 {
 	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);