sound: usb-audio: add workaround for Blue Microphones devices

Blue Microphones USB devices have an alternate setting that sends two
channels of data to the computer.  Unfortunately, the descriptors of
that altsetting have a wrong channel setting, which means that any
recorded data from such a device has twice the sample rate from what
would be expected.

This patch adds a workaround to ignore that altsetting.  Since these
devices have only one actual channel, no data is lost.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>