libata: don't consider 0xff as port empty if SStatus is available

Some SATA controllers (sata_sil) use 0xff to indicate port not ready
status, not port empty.  As libata interprets 0xff as port empty, this
causes unnecessary reset failure and retry.  Don't consider 0xff as
port empty if SStatus is available and indicates that port is online.

Signed-off-by: tejun Heo <htejun@gmail.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Indan Zupancic <indan@nul.nu>
Signed-off-by: Jeff Garzik <jeff@garzik.org>