m68k/mac: Fix apparent race condition in Baboon interrupt dispatch

The algorithm used in baboon_irq() appears to be subject to a race
condition: an IRQ flag could be lost if asserted between the MOV
instructions from and to the interrupt flag register. However,
testing shows that the write to the flag register has no effect.
Rewrite this loop to remove the apparent race condition.

No-one seems to know how to clear Baboon IRQ flags, or whether
that's even possible, so add a comment about this.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>