ALSA: usb-audio: make buffer pointer based on bytes instead on frames

Since there are devices that do not align the size of their data packets
to frame boundaries, the driver needs to be able to keep track of
partial frames.  This patch prepares for support for such devices by
changing the hwptr_done variable from a frame counter to a byte counter.

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