m68k/mac: More printk modernization

Log message fragments used to be printed on one line but now get split up.
Fix this. Also, suppress log spam that merely prints known pointer values.

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>