ALSA: firewire-speakers: fix locking

There is a lock inversion between fwspk->mutex and pcm->open_mutex
reported by lockdep when fwspk_hw_free is called.

Fixed by copying the fix from the same former issue in the isight
sound driver (commit f3f7c183
"ALSA: isight: fix locking").

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>