[patch] libata: ata_piix.c PIO fix

"[libata] transfer mode cleanup" introduced bug in ata_piix.c:
previously PIO number (not mode) was passed to piix_set_piomode().
Fortunately this function is only used for (disabled) PATA support.

I bet that this is the reason why MWDMA didn't work for PATA.
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@elka.pw.edu.pl>