Bluetooth: Initialize tx_win_max for fixed channel

tx_win_max is initialized during L2CAP configuration phase. For fixed
channels (e.g. A2MP) we want to have it initialized when channel is
created.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>