Merge ata_host_set_pio() and ata_host_set_dma() into ata_set_mode()
and use function-level *dev to iterate over devices.  This eases
soon-to-follow ata_set_mode() interface change.

While at it, kill an unnecessary comment and normalize others.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>