PCI: Replace HAVE_ARCH_PCI_MWI with PCI_DISABLE_MWI

pSeries is the only architecture left using HAVE_ARCH_PCI_MWI and it's
really inappropriate for its needs.  It really wants to disable MWI
altogether.  So here are a pair of stub implementations for pci_set_mwi
and pci_clear_mwi.

Also rename pci_generic_prep_mwi to pci_set_cacheline_size since that
better reflects what it does.

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