Use set_rts_threshold calback to enable/disable threshold only for
legacy traffic.

Protection for HT and VHT traffic is defined by HT operation element
and it's provided by remote AP or by hostapd.
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>