powerpc: Lack of ibm,io-events not that important!

The ibm,io-events code is a bit verbose with its error messages.
Reverse the reporting so we only print when we successfully enable
I/O event interrupts.

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