[Bluetooth]: Implement RFCOMM remote port negotiation

This patch implements the remote port negotiation (RPN) of the RFCOMM
protocol for Bluetooth.

Signed-off-by: J. Suter <jsuter@hardwave.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>