at91_ide: turn on PIO 6 support

As we have already PIO 6 transfer mode supported in IDE layer, we can turn
it on in the driver.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Tested-by: "Steve Wootton" <swootton@esi-estech.com>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>