sound: usb-audio: allow period sizes less than 1 ms

To enable periods shorter than 1 ms, we have to make sure that short
periods are only available for alternate settings that have a small
enough data packet interval.  Furthermore, the code that aligns URBs to
USB frames is now superfluous.

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