powerpc: Fix irq_free_virt by adjusting bounds before loop

Instead of looping over each irq and checking against the irq array
bounds, adjust the bounds before looping.

The old code will not free any irq if the irq + count is above
irq_virq_count because the test in the loop is testing irq + count
instead of irq + i.

This code checks the limits to avoid unsigned integer overflows.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>