genirq/ipi-mux: Use irq_domain_alloc_irqs()

Using __irq_domain_alloc_irqs() is an unnecessary complexity. Use
irq_domain_alloc_irqs(), which is simpler and makes the code more
readable.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Marc Zyngier <maz@kernel.org>