libata: kill ata_id_to_dma_mode()

ata_id_to_dma_mode() isn't quite generic.  The function is basically
privately implemented ata_id_xfermask() combined with hardcoded mode
printing and configuration which are specific to ata_generic.

Kill the function and open code it in generic_set_mode() using generic
xfermode handling functions.

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