PCI: dwc: Use PTR_ERR_OR_ZERO to simplify code

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR to avoid the
following warnings found by scripts/coccinelle/api/ptr_ret.cocci:

  drivers/pci/dwc/pcie-qcom.c:215:1-3: WARNING: PTR_ERR_OR_ZERO can be used
  drivers/pci/dwc/pcie-qcom.c:247:1-3: WARNING: PTR_ERR_OR_ZERO can be used
  drivers/pci/dwc/pcie-qcom.c:481:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Kishon Vijay Abraham I <kishon@ti.com>