hw/slavio_intctl.c: fix gcc warning about array bounds overrun

The Ubuntu 10.10 gcc for ARM complains that we might be overrunning
the cpu_irqs[][] array: silence this by correcting the bounds on the
loop. (In fact we would not have overrun the array because bit
MAX_PILS in pil_pending and irl_out will always be 0.)

Also add a comment about why the loop's lower bound is OK.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
