genirq: fix off by one and coding style

Fix off-by-one in for_each_irq_desc_reverse().

Impact is near zero in practice, because nothing substantial wants to
iterate down to IRQ#0 - but fix it nevertheless.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>