x86/ioapic.c: clean up replace_pin_at_irq_node logic and comments

There's no need for a control variable in replace_pin_at_irq_node();
it can just return if it finds the old apic/pin to replace.

If the loop terminates, then it didn't find the old apic/pin, so it can
add the new ones.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>