ALSA: firewire-motu: Protect register DSP event queue positions

ALSA: firewire-motu: Protect register DSP event queue positions

The register DSP event queue is updated under parser->lock, but
snd_motu_register_dsp_message_parser_count_event() reads pull_pos and
push_pos without the lock.
snd_motu_register_dsp_message_parser_copy_event() also reads both queue
positions before taking the lock.

Protect these accesses with parser->lock as well. This keeps the hwdep
poll/read path consistent with the producer side and with the cached
meter/parameter accessors.

Fixes: 634ec0b2906e ("ALSA: firewire-motu: notify event for parameter change in register DSP model")
Cc: stable@vger.kernel.org
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260521-alsa-firewire-motu-event-locking-v1-1-708e1c2b5e56@gmail.com