Bluetooth: L2CAP: Use default TxWindow for zero socket option

L2CAP_OPTIONS accepts a txwin_size of zero and
stores it in tx_win. The zero is then sent to the
peer in our configuration request, and if the
channel ends up in ERTM mode it is passed to
l2cap_seq_list_init().

Use the default TxWindow instead, as the mode can
still be changed to ERTM during configuration.

Fixes: 14b5aa71ec50 ("Bluetooth: Add sockopt configuration for txWindow on L2CAP")
Cc: stable@vger.kernel.org
Signed-off-by: Siwei Zhang <oss@fourdim.xyz>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>