[PATCH] PCI: make it easier to see that set_msi_affinity() is used

I missed this usage in drivers/pci/msi.h:

#ifdef CONFIG_SMP
#define set_msi_irq_affinity    set_msi_affinity
#else
#define set_msi_irq_affinity    NULL
#endif

set_msi_affinity() is declared and exclusively used in msi.c.
Here's a better way so (hopefully) history doesn't repeat itself.

Signed-off-by: Grant Grundler <iod00d@hp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>