V4L/DVB (11340): af9015: add support for AverMedia AVerTV Volar Black HD (A850)

Add USB ID (07ca:850a) and configuration hack for AverMedia AVerTV Volar
Black HD (A850) DVB-T USB stick.

Tested-by: Olivier MENUEL <omenuel@laposte.net>
Tested-by: Thomas Renard <threnard@gmail.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>