Bluetooth: hci_serdev: call init_rwsem() before p->open()

kvartet reported, that hci_uart_tx_wakeup() uses uninitialized rwsem.
The problem was in wrong place for percpu_init_rwsem() call.

hci_uart_proto::open() may register a timer whose callback may call
hci_uart_tx_wakeup(). There is a chance, that hci_uart_register_device()
thread won't be fast enough to call percpu_init_rwsem().

Fix it my moving percpu_init_rwsem() call before p->open().

INFO: trying to register non-static key.
The code is fine but needs lockdep annotation, or maybe
you didn't initialize this object before use?
turning off the locking correctness validator.
CPU: 2 PID: 18524 Comm: syz-executor.5 Not tainted 5.16.0-rc6 #9
...
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 assign_lock_key kernel/locking/lockdep.c:951 [inline]
 register_lock_class+0x148d/0x1950 kernel/locking/lockdep.c:1263
 __lock_acquire+0x106/0x57e0 kernel/locking/lockdep.c:4906
 lock_acquire kernel/locking/lockdep.c:5637 [inline]
 lock_acquire+0x1ab/0x520 kernel/locking/lockdep.c:5602
 percpu_down_read_trylock include/linux/percpu-rwsem.h:92 [inline]
 hci_uart_tx_wakeup+0x12e/0x490 drivers/bluetooth/hci_ldisc.c:124
 h5_timed_event+0x32f/0x6a0 drivers/bluetooth/hci_h5.c:188
 call_timer_fn+0x1a5/0x6b0 kernel/time/timer.c:1421

Fixes: d73e1728 ("Bluetooth: hci_serdev: Init hci_uart proto_lock to avoid oops")
Reported-by: Yiru Xu <xyru1999@gmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>