ALSA: seq: Cover unsubscribe_port() in list_mutex

The call of unsubscribe_port() which manages the group count and
module refcount from delete_and_unsubscribe_port() looks racy; it's
not covered by the group list lock, and it's likely a cause of the
reported unbalance at port deletion.  Let's move the call inside the
group list_mutex to plug the hole.

Reported-by:  <syzbot+e4c8abb920efa77bace9@syzkaller.appspotmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>