powerpc/pseries: Return req#msi(-x) if request is larger

If a driver asks for more MSIs than the devices "req#msi(-x)" property,
we currently return -ENOSPC. This doesn't give the driver any chance to
make a new request with a number that might work.

So if "req#msi(-x)" is less than the request, return its value. To be
100% safe, make sure we return an error if req_msi == 0.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/msi.c

@@ -154,7 +154,11 @@ static int check_req(struct pci_dev *pdev, int nvec, char *prop_name)
 
 	if (*req_msi < nvec) {
 		pr_debug("rtas_msi: %s requests < %d MSIs\n", prop_name, nvec);
-		return -ENOSPC;
+
+		if (*req_msi == 0) /* Be paranoid */
+			return -ENOSPC;
+
+		return *req_msi;
 	}
 
 	return 0;