pata: expose set_mode method so it can be wrapped

This splits set_mode into do_set_mode and the wrapper so that a driver can
call the standard method inside its own.  This in theory also obsoletes
->post_set_mode().

Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: Tejun Heo <htejun@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>