[ALSA] usb-audio support for Turtle Beach Roadie

From: Sam Revitch <sam.revitch@gmail.com>Recently a Turtle Beach Audio Advantage Roadie device ended up in my
possession.  It seems to work with the snd-usb-audio driver, but only
using the headphone jack in 2-channel mode.  The device has a DIN
connector carrying six more channels that are otherwise silent.
C-Media has freely available documentation for the CM106 chip around
which this device is based, and enabling 8-channel output, or
6-channel output with the headphone jack following the front pair is a
matter of setting one of its registers.
Attached is a patch to try to enable 5.1 output mode at probe time.
It seems to work correctly with my device.  There is quite list of
other configurables for this device that might deserve controls.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>