ALSA: usb-audio: Unify the code for the next packet size calculation

There are two places calculating the next packet size for the playback
stream in the exactly same way.  Provide the single helper for this
purpose and use it from both places gracefully.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-32-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>