Bluetooth: Fix missing hci_dev_lock/unlock in hci_event

mgmt_pending_remove() should be called with hci_dev_lock protection and
all hci_event.c functions which calls mgmt_complete() (which eventually
calls mgmt_pending_remove()) should hold the lock.
So this patch fixes the same

Signed-off-by: Jaganath Kanakkassery <jaganath.k@samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>