PCI: Use pci_generic_prep_mwi on ia64

The pci_generic_prep_mwi() code does everything that pcibios_prep_mwi()
does on ia64.  All we need to do is be sure that pci_cache_line_size
is set appropriately, and we can delete pcibios_prep_mwi().

Using SMP_CACHE_BYTES as the default was wrong on uniprocessor machines
as it is only 8 bytes.  The default in the generic code of L1_CACHE_BYTES
is at least as good.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>