sound: usb-audio: make URB sizes more equal

Distribute the packets evenly among the URBs, instead of making all URBs
except the last one to have the maximum size.  This makes the timing of
pointer updates more regular and removes some special cases from the
code.

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