ide-pmac: remove nIEN clearing from pmac_ide_do_setfeature()

Upper layers are responsible for controlling nIEN so don't clear nIEN after
command execution in pmac_ide_do_setfeature().

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>