PCI/AER: Move internal declarations to drivers/pci/pci.h

Since pci_aer_init() and pci_no_aer() are used only internally, move their
declarations to the PCI internal header file.  Also, no one cares about
return value of pci_aer_init(), so make it void.

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