Bluetooth: Store RFCOMM address information in its own socket structure

The address information of RFCOMM sockets should be stored in its
own socket structure. Trying to generalize them is not helpful since
different transports have different address types.

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