V4L/DVB (8017): Ensure em28xx extensions only get run against devs that support them

em28xx-audio.c
em28xx-dvb.c
 - Em28xx extensions should ensure they are being only loaded against devices
   that support them.  Deals with case where there are multiple em28xx
   devices, some of which have DVB (or ALSA) support and some do not.

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>