wl18xx: split siso40 HT cap between 2Ghz and 5Ghz

Remove the cap IEEE80211_HT_CAP_DSSSCCK40 from the 5Ghz variant of
the siso40 HT capabilities. It is meaningless in 5Ghz.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>