mt76: Fix a signedness bug in mt7615_add_interface()

The problem is that "mvif->omac_idx" is a u8 so it can't be negative
and the error handling won't work.  The get_omac_idx() function returns
-1 on error.

Fixes: 04b8e659 ("mt76: add mac80211 driver for MT7615 PCIe-based chipsets")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7615/main.c

@@ -77,11 +77,12 @@ static int mt7615_add_interface(struct ieee80211_hw *hw,
 		goto out;
 	}
 
-	mvif->omac_idx = get_omac_idx(vif->type, dev->omac_mask);
-	if (mvif->omac_idx < 0) {
+	idx = get_omac_idx(vif->type, dev->omac_mask);
+	if (idx < 0) {
 		ret = -ENOSPC;
 		goto out;
 	}
+	mvif->omac_idx = idx;
 
 	/* TODO: DBDC support. Use band 0 and wmm 0 for now */
 	mvif->band_idx = 0;