Bluetooth: Convert hci_cb_list_lock to a mutex

We'll soon need to be able to sleep inside the loops that iterate the
hci_cb list, so neither a spinlock, rwlock or rcu are usable. This patch
changes the lock to a mutex which permits sleeping while holding the
lock.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>