ALSA: usb-audio: Replace slave/master terms

Follow the inclusive terminology, just replace sync_master/sync_slave
with sync_source/sync_sink.  It's also a bit clearer from its meaning,
too.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-34-tiwai@suse.de


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