sound: usb-audio: use multiple input URBs

Some newer USB MIDI interfaces use rather small packet sizes, so to get
enough bandwidth, we have to be able to receive multiple packets in one
USB frame, so we have to use multiple URBs.

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