Bluetooth: Fix HCIUARTGETDEVICE ioctl when UART is not registered

The protocol for the UART might be configured, but that does not
mean the HCI device is registered. Return an error in that case
and only return the index number when HCI_UART_REGISTERED is set.

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