libata: clean up xfermode / PATA timing related stuff

* s/ATA_BITS_(PIO|MWDMA|UDMA)/ATA_NR_\1_MODES/g

* Consistently use 0xff to indicate invalid transfer mode (0x00 is
  valid for PIO_SLOW).

* Make ata_xfer_mode2mask() return proper mode mask instead of just
  the highest bit.

* Sort ata_timing table in increasing xfermode order and update
  ata_timing_find_mode() accordingly.

This patch doesn't introduce any behavior change.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>