[ALSA] usb-audio: schedule high speed URBs with 1 ms alignment

USB generic driver
The EHCI driver doesn't interrupt more than once per millisecond, and
organizes all iso transfers with frame-sized ITDs, so we can (try to)
be more efficient by aligning all URBs on frame boundaries.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>