[ALSA] usb-audio: double-buffer all playback data

USB generic driver
We always had to use double buffering when capturing, and when playback
data for one URB crosses a buffer boundary.  The latter would make hwptr
updates less precise because the double-buffered data is read from the
buffer much earlier than the other data is read by the host controller.

Double-buffering all data allows to update hwptr immediately after the
data was copied to the USB buffer(s), which has the additional benefit
of avoiding the latency imposed by the host controller's delay of up to
one frame when interrupting.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>