powerpc/irq: Quieten irq mapping printks

HFI creates interrupts each time a window is setup. This results in
a lot of messages in the kernel log buffer:

    irq: irq 199007 on host null mapped to virtual irq 351

This box has over 3500 of them, causing more important kernel
messages to be overwritten. We can get at this information via
debugfs now so we may as well turn it into a pr_debug.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>