PCI: Add check code for last image indicator not set

If the "last image" indicator was not set in the PCI data struct, print "No
more image in the PCI ROM" instead of looping back and printing "Invalid
PCI ROM header signature".

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>