libata-sff: reorder SFF/BMDMA functions

Reorder functions such that SFF and BMDMA functions are grouped.
While at it, s/BMDMA/SFF in a few comments where it actually meant
SFF.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>