wifi: mt76: mt7915: fix mt7915_rate_txpower_get() resource leaks

Coverity message: variable "buf" going out of scope leaks the storage.
Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1527799 ("Resource leaks")
Fixes: e3296759 ("wifi: mt76: mt7915: enable per bandwidth power limit support")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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

@@ -996,7 +996,7 @@ mt7915_rate_txpower_get(struct file *file, char __user *user_buf,
 
 	ret = mt7915_mcu_get_txpower_sku(phy, txpwr, sizeof(txpwr));
 	if (ret)
-		return ret;
+		goto out;
 
 	/* Txpower propagation path: TMAC -> TXV -> BBP */
 	len += scnprintf(buf + len, sz - len,
@@ -1047,6 +1047,8 @@ mt7915_rate_txpower_get(struct file *file, char __user *user_buf,
 			 mt76_get_field(dev, reg, MT_WF_PHY_TPC_POWER));
 
 	ret = simple_read_from_buffer(user_buf, count, ppos, buf, len);
+
+out:
 	kfree(buf);
 	return ret;
 }