Bluetooth: Simplify RFCOMM session state eval

Merge conditional test for BT_LISTEN session state into following
switch statement (which is functionally equivalent).

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Tested-By: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>