sata_sis: convert to slave_link

During introduction of slave_link, sata_sis slipped through the crack
and left with ad-hoc merged SCR access.  As SCR status was shared for
both the master and slave devices, when only one of the device is
online, libata EH would think both are online but would only get valid
device signature for the actually present one, which in turn trigger
the probing safety net mechanism and make EH retry causing large delay
during boot.  This patch converts sata_sis to slave_link mechanism.

This bug was reported by TAXI in bko#14075.

 http://bugzilla.kernel.org/show_bug.cgi?id=14075

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: TAXI <taxi@a-city.de>
Cc: Uwe Koziolek <uwe.koziolek@gmx.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>