PCI: rcar: Fix rcar_pci_probe() return value check

In case of error, the function devm_ioremap_resource() returns ERR_PTR()
and never returns NULL.  The NULL test in the return value check should
be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>