libata-sff: handle controllers w/o ctl register

SFF incorrectly assumed that ctl register is available for all
controllers while some old SFF controllers don't have ctl register.
Make SFF handle controllers w/o ctl register by conditionalizing ctl
register access and softreset method.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jeff Garzik <jeff@garzik.org>