m68k/mac: Don't hang waiting for Cuda power-down command

Testing shows that the CUDA_POWERDOWN command never completes (for a
Centris 660av or LC 475, at least). So, don't wait for command completion
on those Cuda-based models that do not support soft power. Just proceed to
log the usual message, "It is now safe to turn off your Macintosh."

Reported-by: Stan Johnson <userm57@yahoo.com>
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>