ide-pmac: use custom hwif->sg_max_nents only if DMA support is enabled

Move setting hwif->sg_max_nents from pmac_ide_setup_device()
to pmac_ide_setup_dma().

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