media: mc-entity.c: use WARN_ON, validate link pads

Use WARN_ON instead of BUG_ON.

Add two new WARN_ONs to verify that the source pad is really a source
and that the sink pad is really a sink.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: use ! instead of == NULL for source and sink]
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>