PCI: Create alloc_pci_dev(), the one true way to create a struct pci_dev

There are currently several places in the kernel where we kmalloc()
a struct pci_dev and start initialising it. It'd be preferable to
have an allocator so we can ensure the pci_dev is correctly initialised
in one place.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>