ide-pmac: IDE cable detection on Apple PowerBook

As IDE cable used on Apple PowerBook/iBook laptops are always of "Short 40"
type when the firmware says it's 80 conductor one, the cable detection should
return ATA_CBL_PATA40_SHORT on those machines.  This enables to automatically
use UDMA5 even with drives that doesn't correctly detect those cables on Apple
laptops.

Signed-off-by: TOMARI Hisanobu <posco.grubb@gmail.com>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: benh@kernel.crashing.org
[bart: beautify patch description]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>