Bluetooth: Refuse peer RFCOMM address reading when not connected

When we're not connected the peer address information is undefined. This
patch fixes the remote address getting to return a proper error in case
the sate is anything else than BT_CONNECTED.

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