ata_piix: fix pio/mwdma programming

Fix various bugs in pio/mwdma mode programming.

* Control bits in the timing register wasn't cleared properly while
  programming PIO mode.

* MWDMA mode programming cleared the wrong part of control bits.

* MWDMA mode programming cleared udma_mask even when the controller
  doesn't support UDMA.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Art Haas <ahaas@airmail.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>