Fix NF calibration breakage introduced by me in a past commit.

Since the returned NF will be -ve, checking for <= 0 is not good
enough. For now, check whether it equals 0 or -1; a future commit
will tidy this mess up and have it return HAL_BOOL instead.

sys/dev/ath/ath_hal/ar5416/ar5416_cal.c

@@ -501,7 +501,7 @@ ar5416PerCalibrationN(struct ath_hal *ah, struct ieee80211_channel *chan,
 		 * and update the history buffer.
 		 */
 		r = ar5416GetNf(ah, chan);
-		if (r <= 0) {
+		if (r == 0 || r == -1) {
 			/* NF calibration result isn't valid */
 			HALDEBUG(ah, HAL_DEBUG_UNMASKABLE, "%s: NF calibration"
 			    " didn't finish; delaying CCA\n", __func__);