wifi: mt76: fix coverity uninit_use_in_call in mt76_connac2_reverse_frag0_hdr_trans()

The default case for frame_contorl is invalid. We should always
assign addr3 of this frame properly.

Coverity error message:
if (ieee80211_has_a4(hdr.frame_control))
(19) Event uninit_use_in_call:	Using uninitialized value "hdr".
Field "hdr.addr3" is uninitialized when calling "memcpy".
	memcpy(skb_push(skb, sizeof(hdr)), &hdr, sizeof(hdr));
else
	memcpy(skb_push(skb, sizeof(hdr) - 6), &hdr, sizeof(hdr) - 6);

Fixes: 0880d408 ("mt76: connac: move mt76_connac2_reverse_frag0_hdr_trans in mt76-connac module")
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt76_connac_mac.c

@@ -930,7 +930,7 @@ int mt76_connac2_reverse_frag0_hdr_trans(struct ieee80211_vif *vif,
 		ether_addr_copy(hdr.addr4, eth_hdr->h_source);
 		break;
 	default:
-		break;
+		return -EINVAL;
 	}
 
 	skb_pull(skb, hdr_offset + sizeof(struct ethhdr) - 2);