m68k/mac: Fix mac_irq_pending() for PSC MACE and SCC

Add missing return statement. The docs say that the level 4 PSC IRQs
relate to MACE DMA and SCC. Since those drivers don't call
mac_irq_pending() this patch has no affect. But it should be fixed all the
same, since it can be useful for MACE debugging.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>