V4L/DVB (6539): em28xx: add support for vgear pockettv

attached patch adds support for the vgear pockettv.
It seems to require a write to another register for audio to work.
I checked my old cinergydrv and we did the same register write there. I
therefore enabled it for all em2800 based devices.

Signed-off-by: Sascha Sommer <saschasommer@freenet.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>