Bluetooth: SCO: remove the redundant sco_conn_put

When adding conn, it is necessary to increase and retain the conn reference
count at the same time.

Another problem was fixed along the way, conn_put is missing when hcon is NULL
in the timeout routine.

Fixes: e6720779 ("Bluetooth: SCO: Use kref to track lifetime of sco_conn")
Reported-and-tested-by:  <syzbot+489f78df4709ac2bfdd3@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=489f78df4709ac2bfdd3


Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>