[PATCH] libata: reimplement ata_set_mode() using xfer_mask helpers

Use xfer_mask helpers to determine transfer mode.  This rewrite also
makes transfer mode determination done before any actual
configuration.  This patch doesn't result in any functional changes.

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