PCI: fix section mismatch warnings referring to pci_do_scan_bus

Fix following warnings:
WARNING: o-x86_64/drivers/pci/built-in.o(.text+0xb054): Section mismatch in reference from the function cpci_configure_slot() to the function .devinit.text:pci_do_scan_bus()
WARNING: o-x86_64/drivers/pci/built-in.o(.text+0x153ab): Section mismatch in reference from the function shpchp_configure_device() to the function .devinit.text:pci_do_scan_bus()
WARNING: o-x86_64/drivers/pci/built-in.o(__ksymtab+0xc0): Section mismatch in reference from the variable __ksymtab_pci_do_scan_bus to the function .devinit.text:pci_do_scan_bus()

PCI hotplug were the only user of pci_do_scan_bus()
so moving this function to a separate file that is build
only when we enable CONFIG_HOTPLUG_PCI.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>