m68k: Revive reporting of spurious interrupts

commit 2502b667 ("Change the m68knommu irq
handling to use the generic irq framework.") removed the reporting of spurious
interrupts on nommu (68328 and 68360).

Bring it back in a generic way, using "atomic_t irq_err_count", as that's what
most of the other architectures are using.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>