Now that we have ata_do_set_mode() available for drivers to use we don't
actually need ->post_set_mode() as the driver can wrap set_mode nicely
and do stuff before or after (eg PCMCIA needs before), so we can kill off
a method in all the structs

While I was at it I added kernel-doc to the function involved.
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>