genirq: Remove redundant NULL check of irq_desc

for_each_irq_desc() macro has already skipped NULL irq_desc, don't
bother to check it again.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Cc: mingo@kernel.org
Cc: yhlu.kernel@gmail.com
Link: http://lkml.kernel.org/r/1458395959-7046-1-git-send-email-nasa4836@gmail.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>