ALSA: usb-audio: add support for samplerate setting on v2 devices

Sample rate setting is done with a 4-byte long class request that
addresses the interface.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>