genirq/proc: Avoid uninitalized variable warning

kernel/irq/proc.c: In function ‘show_irq_affinity’:
include/linux/cpumask.h:24:29: warning: ‘mask’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 #define cpumask_bits(maskp) ((maskp)->bits)

gcc is silly, but admittedly it can't know that this won't be called with
anything else than the enumerated constants.

Shut up the warning by creating a default clause.

Fixes: 6bc6d4ab ("genirq/proc: Use the the accessor to report the effective affinity
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>