PCI: Index IOV resources in the conventional style

Most of PCI uses "res = &dev->resource[i]", not "res = dev->resource + i".
Use that style in iov.c also.

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>