media: mc-entity.c: use & to check pad flags, not ==

These are bits so to test if a pad is a sink you use & but not ==.

It looks like the only reason this hasn't caused problems before is that
media_get_pad_index() is currently only used with pads that do not set the
MEDIA_PAD_FL_MUST_CONNECT flag. So a pad really had only the SINK or SOURCE
flag set and nothing else.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: <stable@vger.kernel.org>      # for v5.3 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>