The hci_dev struct hdev is referenced in work queues and timers started
by open() in some protocols. This creates a race between the
initialization function and the work or timer which can result hdev
being dereferenced while it is still null.

The syzbot report contains a reliable reproducer which causes a null
pointer dereference of hdev in hci_uart_write_work() by making the
memory allocation for hdev fail.

To fix this, ensure hdev is valid from before calling a protocol's
open() until after calling a protocol's close().

Reported-by: syzbot+257790c15bcdef6fe00c@syzkaller.appspotmail.com
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>