ALSA: sound/usb/usx2y: fix sparse warning: do-while statement is not a compound ...

Fix this sparse warning:
  sound/usb/usx2y/usbusx2y.c:231:33: warning: do-while statement is not a compound statement

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>