ide-pmac: add PIO autotune fallback to ->ide_dma_check

Since ide-pmac allows separate PIO and DMA timings, and ide-pmac always sets
->autotune this change shouldn't have any other effect than bringing ide-pmac
driver in-line with other host drivers wrt ->ide_dma_check implementation.

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