wl18xx: explicitly remove the 5Ghz MIMO HT cap

The 18xx chip does not support MIMO in 5Ghz. Use the siso20 HT cap as
fallback in 5Ghz when "mimo" is requested.

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