Blackfin: fix sparseirq/kstat_irqs fallout

The sparseirq changes (d7e51e66

) played poorly with the Blackfin irqchip
implementation as we're still using the old hardirq method.  Our bad irq
structure had a NULL kstat_irqs field so when all the common code tries
to increment this field, everything goes big bada boom.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>