wlcore: set wl->ht_cap per-band

Save the ht_cap IE per-band, so we can configure different
params to BG and A bands (we currently don't support MIMO
on A band)

[Small fix for rx_highest - Arik]

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