stmmac: pci: use defined constant instead of magic number

The last standard PCI resource is defined as PCI_STD_RESOURCE_END. Thus, we
could use it instead of plain integer.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>