wifi: mt76: mt7915: fix an uninitialized variable bug

Smatch complains that:

    drivers/net/wireless/mediatek/mt76/mt7915/mac.c:428 mt7915_mac_fill_rx()
    error: uninitialized symbol 'msta'.

It looks like this was supposed to be initialized to NULL.

Fixes: 0880d408 ("mt76: connac: move mt76_connac2_reverse_frag0_hdr_trans in mt76-connac module")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/mac.c

@@ -232,7 +232,7 @@ mt7915_mac_fill_rx(struct mt7915_dev *dev, struct sk_buff *skb)
 	bool unicast, insert_ccmp_hdr = false;
 	u8 remove_pad, amsdu_info;
 	u8 mode = 0, qos_ctl = 0;
-	struct mt7915_sta *msta;
+	struct mt7915_sta *msta = NULL;
 	bool hdr_trans;
 	u16 hdr_gap;
 	u16 seq_ctrl = 0;