[MAC80211]: fix preamble setting

It looks like in commit 28487a90 the condition was unintentionally
negated by moving some code, fix it.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Daniel Drake <dsd@gentoo.org>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mac80211/ieee80211_ioctl.c

@@ -821,7 +821,7 @@ static int ieee80211_ioctl_prism2_param(struct net_device *dev,
 		break;
 
 	case PRISM2_PARAM_PREAMBLE:
-		if (sdata->type != IEEE80211_IF_TYPE_AP) {
+		if (sdata->type == IEEE80211_IF_TYPE_AP) {
 			if (value)
 				sdata->flags |= IEEE80211_SDATA_SHORT_PREAMBLE;
 			else