Bluetooth: hci_uart: add missing NULL check in h5_enqueue

Syzbot hit general protection fault in __pm_runtime_resume(). The problem
was in missing NULL check.

hu->serdev can be NULL and we should not blindly pass &serdev->dev
somewhere, since it will cause GPF.

Reported-by:  <syzbot+b9bd12fbed3485a3e51f@syzkaller.appspotmail.com>
Fixes: d9dd833c

 ("Bluetooth: hci_h5: Add runtime suspend")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>