forked from Imagelibrary/rtems
* ep1a/irq/irq.c, gen5200/irq/irq.c, mbx8xx/irq/irq.c, mpc8260ads/irq/irq.c, mvme5500/irq/irq.c, psim/irq/irq.c, score603e/irq/irq.c, shared/irq/irq.c: test for non-NULL-ness before calling 'on'/'off' methods so that users don't have to provide no-ops if they don't want this feature.