wl18xx: support 2 rx stbc streams

The wl18xx chip supports 2 rx stbc streams.
Declare this support in the ht_cap attributes.

Signed-off-by: Eliad Peller <eliad@wizery.com>
wlcore: declare support for ht40 dfs as well (squash)

this should be squashed into the iface combination patch

Signed-off-by: Eliad Peller <eliad@wizery.com>