wl18xx: default to siso40 in 2.4ghz with a single antenna

The driver used siso20 in this case for legacy reasons.

Reported-by: Ido Reis <idor@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>