sata_dwc_460ex: fix misuse of ata_get_cmd_descript()

The driver erroneously uses ata_get_cmd_descript() not only for printing out
the ATA commands but also the protocol and DMA direction enums. Add functions
for properly printing those out...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>