sata_sil24 softreset routine used to check sata_dev_present() after
SRST is complete in the hope that SRST may do some good even when
SStatus reports no device.  This is okay as long as SRST timeout is
short (> 100ms in the current code) but it seems that not all SATA
devices are happy with short SRST timeout.

This patch makes softreset exit early without performing actual SRST
if SStatus reports no device in preparation for lengthening SRST
timeout.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>