ahci: sis controllers actually can do PMP

SIS controllers were blacklisted for PMP as enabling it made device
detection fail whether the device was PMP or not - the natural
conclusion was the controller chokes on SRST w/ pmp==15.  However, it
turned out that the controller just didn't like issuing SRST after
hardreset w/o clearing SError first.  Interestingly, the SRST itself
succeeds but the following commands fail.

If SError is cleared between hardreset and SRST, which is the default
behavior now, everything works fine and SIS controllers work with PMPs
happily.

Remove PMP blacklisting for SIS AHCIs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Piter PUNK <piterpunk@slackware.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>