wl18xx: declare support for greenfield ht_cap

The 18xx fw supports greenfield so add the
IEEE80211_HT_CAP_GRN_FLD flag to the supported
ht capabilities flags.

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