[media] pwc: fix WARN_ON

If start_streaming fails, then the buffers must be given back to vb2 with state
QUEUED, not ERROR. Otherwise a WARN_ON will be generated.

In the disconnect it is pointless to call pwc_cleanup_queued_bufs() as stop_streaming()
will be called anyway.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>