ALSA: snd-usb-caiaq: Missing lock around use of buffer positions

Fix a race which causes snd_pcm_update_hw_ptr_pos() to report a bug.

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