ide-pmac: skip conservative PIO "downgrade"

We can skip conservative PIO "downgrade" (PIO3 becomes PIO2 etc.) on PMAC.

Problem reported by Mikael.

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

drivers/ide/ppc/pmac.c

@@ -1138,6 +1138,7 @@ pmac_ide_setup_device(pmac_ide_hwif_t *pmif, ide_hwif_t *hwif)
 	hwif->drives[0].autotune = IDE_TUNE_AUTO;
 	hwif->drives[1].autotune = IDE_TUNE_AUTO;
 	hwif->host_flags = IDE_HFLAG_SET_PIO_MODE_KEEP_DMA |
+			   IDE_HFLAG_PIO_NO_DOWNGRADE |
 			   IDE_HFLAG_POST_SET_MODE;
 	hwif->pio_mask = ATA_PIO4;
 	hwif->set_pio_mode = pmac_ide_set_pio_mode;