m68k/mac: Enable PDMA support for PowerBook 190

Stan's tests showed that PDMA improves sequential read performance by
a factor of 5 on a PowerBook 190. Last time I tried this on a
PowerBook 520 it didn't work, so let's not enable it there until
it can be tested with the present mac_scsi driver.

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>