wl1271: Fix mac80211 RTS threshold requests during WL1271_STATE_OFF

mac80211 sends RTS threshold configuration request even if the wl1271 interface
state is WL1271_STATE_OFF. This leads to failures during pm tests.

This patch leaves the configuration function, if the interface is
going down.

Signed-off-by: Saravanan Dhanabal <ext-saravanan.dhanabal@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>