Bluetooth: Enable option to configure Max Transmission value via sockopt

With the sockopt extension we can set a per-channel MaxTx value.

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>