doc: Add missing parameter for msi_setup

commit 92ca8d20dee2 ("genirq/msi: Switch to new irq spreading")
introduced new parameter to msi_init_setup and but did not update
docbook comments. Fixes 'make htmldocs' warning.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Cc: bhelgaas@google.com
Cc: linux-pci@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>