V4L/DVB (9101): radio-mr800: Add driver for AverMedia MR 800 USB FM radio devices

This patch creates a new usb-radio driver, radio-mr800.c, that
supports the AverMedia MR 800 USB FM radio devices.
This device plugs into both the USB and an analog audio input, so this
thing only deals with initialization and frequency setting, the audio
data has to be handled by a sound driver.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>