PCI: designware: Rename dw_pcie_valid_config() to dw_pcie_valid_device()

Rename dw_pcie_valid_config() to dw_pcie_valid_device() and use the result
directly as a boolean value instead of testing against 0.  No functional
change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>