ide-timings: use ->pio_mode value to determine fastest PIO speed

Use the current PIO mode value instead of the physical maximum one
to determine the fastest allowed PIO for shared PIO/DMA timings.

Affected host drivers: amd74xx and via82cxxx.

[ Update comment to match -DaveM ]

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>