genirq: Fix type inconsistency

The return type of kstat_irqs_usr() is unsigned int and kstat_irqs() also
returns unsigned int so sum should be unsigned int here as well. 

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Link: http://lkml.kernel.org/r/1430642951-23964-1-git-send-email-hofrat@osadl.org


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