Add pio_mode field to ide_drive_t matching pio_mode field used in
struct ata_device.

The validity of the field is restricted to ->set_pio_mode method
only currently in IDE subsystem.
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>