Bluetooth: Add sockopt configuration for txWindow on L2CAP

Now we can set/get Transmission Window size via sockopt.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>