PCI: Print warnings for all invalid expansion ROM headers

We've always validated that both bytes of the Expansion ROM signature and
all four bytes of the PCI Data Structure signature (see PCI Firmware spec
r3.0, sec 5.1.1), but we only printed a warning if the first byte of the
ROM signature was invalid.

Print warnings if *any* of those bytes are invalid.  Note that we only look
at these headers if we map or read the ROM.

[bhelgaas: changelog, tweak printk format]
Signed-off-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>