hostapd: enable STBC only for STBC capable STAs

hostapd: enable STBC only for STBC capable STAs

hostapd simply used its own STBC configuration in the STA's HT caps. This
resulted in TX STBC being used for STAs not supporting RX STBC, which in
turn resulted in the STA not receiving anything.

Fix this by handling the STBC flags in the same way mac80211 does. Mask
out RX STBC if we don't support TX STBC and vice versa.

Tested only with the nl80211 driver and a STBC incapable STA.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>