[media] tvp5150: Fix breakage for serial usage

changeset 460b6c08

 ("tvp5150: Add s_stream subdev operation
support") broke for em28xx-based devices with uses tvp5150. On those
devices, touching the TVP5150_MISC_CTL register causes em28xx to stop
streaming.

I suspect that it uses the 27 MHz clock provided by tvp5150 to feed
em28xx. So, change the logic to do nothing on s_stream if the tvp5150 is
not set up to work with V4L2_MBUS_PARALLEL.

Tested with Hauppauge WinTV USB 2 model 42012 Rev. C186
(USB ID: 2040:4200).

Cc: Javier Martinez Canillas <javier@osg.samsung.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>