genirq/msi: Return MSI_XA_DOMAIN_SIZE as the maximum MSI index when no domain is present

On architectures such as s390 that do not use irq domains for MSI,
returning 0 as the maximum MSI index is a bit counter-productive,
as it indicates that no MSI can be allocated. Bad idea.

Instead, return the maximum we're willing to support in the MSI
backing store (MSI_XA_DOMAIN_SIZE), and let the arch code do its
usual thing.

Thanks to Matthew Rosato for fixing the fix.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[maz: commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/87fsdgzpqs.ffs@tglx