ALSA: mixer: fix deadlock in snd_mixer_oss_set_volume

ALSA: mixer: fix deadlock in snd_mixer_oss_set_volume

In commit 411cef6adfb3 ("ALSA: mixer: oss: Fix racy access to slots")
added mutex protection in snd_mixer_oss_set_volume(). Second
mutex_lock() in same function looks like typo, fix it.

Reported-by: syzbot+ace149a75a9a0a399ac7@syzkaller.appspotmail.com
Fixes: 411cef6adfb3 ("ALSA: mixer: oss: Fix racy access to slots")
Cc: <stable@vger.kernel.org>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Link: https://lore.kernel.org/r/20211024140315.16704-1-paskripkin@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>