Bluetooth: Add initial code for LE L2CAP Connect Request

This patch adds the necessary code to send an LE L2CAP Connect Request
and handle its response when user space has provided us with an LE
socket with a PSM instead of a fixed CID.

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