ata_generic: drop hard coded DMA force logic for CENATEK

Commit 1529c69a (ata_generic: implement ATA_GEN_* flags and force
enable DMA on MBP 7,1) implemented ATA_GEN_FORCE_DMA for forcing DMA
mode and applied it to CENATEK but forgot to remove the original hard
coded logic.  This is removal of redundant logic and doesn't affect
correctness.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>