octeontx2-pf: Fix an IS_ERR() vs NULL bug

The otx2_mbox_get_rsp() function never returns NULL, it returns error
pointers on error.

Fixes: 34bfe0eb ("octeontx2-pf: MTU, MAC and RX mode config support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/marvell/octeontx2/nic/otx2_common.c

@@ -171,9 +171,9 @@ static int otx2_hw_get_mac_addr(struct otx2_nic *pfvf,
 	}
 
 	msghdr = otx2_mbox_get_rsp(&pfvf->mbox.mbox, 0, &req->hdr);
-	if (!msghdr) {
+	if (IS_ERR(msghdr)) {
 		otx2_mbox_unlock(&pfvf->mbox);
-		return -ENOMEM;
+		return PTR_ERR(msghdr);
 	}
 	rsp = (struct nix_get_mac_addr_rsp *)msghdr;
 	ether_addr_copy(netdev->dev_addr, rsp->mac_addr);