ALSA: 6fire: take stream_mutex in the Digital Thru control

usb6fire_control_digital_thru_put() calls
usb6fire_control_streaming_update(), which on an idle card with Digital
Thru on selects altsetting 2 and writes the clock register for 96 kHz
through usb6fire_control_set_rate(). usb6fire_pcm_prepare() calls both
functions through usb6fire_pcm_set_rate() under rt->stream_mutex. The
put does not take stream_mutex, so the two can run at the same time.

500 runs of a 0.2 s aplay at 48 kHz while a second loop toggled
"Digital Thru Playback Route" with amixer (12693 on/off cycles, kernel
7.2.9): in 216 cases the two usb_set_interface() calls ran at the same
time and the kernel logged "sysfs: cannot create duplicate filename
'.../3-2:1.1/ep_82'" with a stack dump. 27 of the aplay runs failed,
and for each of them the kernel logged "could not start pcm stream".

Take stream_mutex in the put. The pcm runtime is created before the
controls and freed with the card, so chip->pcm is valid whenever the
put runs. The put waits for a running prepare, which makes synchronous
USB transfers under stream_mutex; on a card that responds late the wait
can reach about 12 s, or 24 s with Digital Thru on, and during the wait
the other controls of the card can be neither read nor written, because
snd_ctl_elem_write() holds controls_rwsem around the put. Before this
patch the put already made transfers of the same kind under
controls_rwsem. No code that holds stream_mutex takes controls_rwsem.

The same test on kernel 7.2.10 with this patch (12863 cycles) gave no
failed aplay run and no "duplicate filename" message.

Fixes: 2475b0d40761 ("ALSA: 6fire - Add support of digital-thru mixer")
Assisted-by: LLM
Signed-off-by: Piotr Gliźniewicz <p.glizniewicz@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20261016073132.109288-2-p.glizniewicz@gmail.com

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