libata-sff: add ata_sff_queue_work() & ata_sff_queue_delayed_work()

This patch adds ata_sff_queue_work() & ata_sff_queue_delayed_work() routine in
libata-sff.c file. This routine can be used by ata drivers to use ata_sff_wq.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>