powerpc/xive: Ease debugging of xive_irq_set_affinity()

pr_debug() is easier to activate and it helps to know how the kernel
configures the HW when tweaking the IRQ subsystem.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-5-clg@kaod.org