parisc/irq: Simplify irq count output for /proc/interrupts

The SMP variant works perfectly fine on UP as well.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-parisc@vger.kernel.org
Link: https://lore.kernel.org/r/20201210194043.172893840@linutronix.de