V4L/DVB (8147): cxusb: add initial support for AVerTVHD Volar

Add initial support for this device. The AGC and tracking filter
settings for the mxl5003 are known to be wrong, but it generally works.

Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>