m68k: mac baboon interrupt enable/disable

No-one seems to know how to mask individual baboon interrupts, so we just 
mask the umbrella IRQ. This will work as long as only the IDE driver uses 
the baboon chip (it can't deadlock). Use mac_enable_irq/mac_disable_irq 
rather than enable_irq/disable_irq because the latter routines count the 
depth of nested calls which triggers a warning and call trace because 
IRQ_NUBUS_C is enabled twice in a row (once when the baboon handler is 
registered, and once when the IDE IRQ is registered).

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