PCI: Improve __pci_read_base() robustness

Local variables 'l' and 'sz' are uninitialized.  Normally, they would
be initialized by pci_read_config_dword() but when an error occurs,
some drivers immediately return an error code, which leaves the
argument uninitialized.

Provide a safe initial value to make the code more robust.

Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>