While handling the HCI_EV_HARDWARE_ERROR event, if the underlying
BT controller is not responding, the GPIO reset mechanism would
free the hci_dev and lead to a use-after-free in hci_error_reset.

Here's the call trace observed on a ChromeOS device with Intel AX201:
   queue_work_on+0x3e/0x6c
   __hci_cmd_sync_sk+0x2ee/0x4c0 [bluetooth <HASH:3b4a6>]
   ? init_wait_entry+0x31/0x31
   __hci_cmd_sync+0x16/0x20 [bluetooth <HASH:3b4a 6>]
   hci_error_reset+0x4f/0xa4 [bluetooth <HASH:3b4a 6>]
   process_one_work+0x1d8/0x33f
   worker_thread+0x21b/0x373
   kthread+0x13a/0x152
   ? pr_cont_work+0x54/0x54
   ? kthread_blkcg+0x31/0x31
    ret_from_fork+0x1f/0x30

This patch holds the reference count on the hci_dev while processing
a HCI_EV_HARDWARE_ERROR event to avoid potential crash.

Fixes: c7741d16 ("Bluetooth: Perform a power cycle when receiving hardware error event")
Signed-off-by: Ying Hsu <yinghsu@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>