ALSA: snd-usb-caiaq: Fix the return of XRUN

Commit 3702b08 added a lock, but did not account for the case of
SNDRV_PCM_POS_XRUN, which would get immediately overwritten.

This could be bundled into one if-else-if statement, but the goto
helps to clarify the 'exceptional' case.

Thanks to Andreas Pape for spotting this.

Signed-off-by: Mark Hills <mark@pogo.org.uk>
Acked-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>