parisc/irq: Use irq_desc_kstat_cpu() in show_interrupts()

The irq descriptor is already there, no need to look it up again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-parisc@vger.kernel.org
Link: https://lore.kernel.org/r/20201210194043.659522455@linutronix.de