wl12xx: Correctly set up protection if non-GF STAs are present

Set the gf_protection bit when calling ACX_HT_BSS_OPERATION according
to the GF bit passed by mac80211 in ht_operation_mode.

[Added a proper commit message -- Luca]

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>