Bluetooth: Change RFCOMM to use BT_CONNECT2 for BT_DEFER_SETUP

When BT_DEFER_SETUP is enabled on a RFCOMM socket, then switch its
current state from BT_OPEN to BT_CONNECT2. This gives the Bluetooth
core a unified way to handle L2CAP and RFCOMM sockets. The BT_CONNECT2
state is designated for incoming connections.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>