[SCSI] isci: link speeds default to gen 2

Gen-3 operation is marginal, default to gen-2 for now.

Signed-off-by: Jeff Skirvin <jeffrey.d.skirvin@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>