isdn: hisax: Replace the bogus access to irq stats

Abusing irq stats in a driver for counting interrupts is a horrible
idea and not safe with shared interrupts. Replace it by a local
interrupt counter.

Noticed by the attempt to remove the irq stats export.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@elte.hu>