Bluetooth: Host: Clear random address field in bt_dev in bt_disable()

This commit clears cached random address in bt_dev when calling
bt_disable(). This change makes future calls of set_random_address()
function possible with previously used address value, after BLE stack
re-initialization. Without this change no HCI command was sent, see this
condition in set_random_address():

/* Do nothing if we already have the right address */
if (!bt_addr_cmp(addr, &bt_dev.random_addr.a)) {
	return 0;
}

Signed-off-by: Adam Augustyn <adam.augustyn@hidglobal.com>