sata_sis: SCR accessors return -EINVAL when requested SCR isn't available

sata_sis: SCR accessors return -EINVAL when requested SCR isn't available

sis_scr_cfg_read() can't access SError and was incorrectly returning
-1 instead of -EINVAL.  This went unnoticed because SError used to be
cleared in @postreset() and it didn't care about how scr_read() failed
but commit ac371987 moved SError clearing into sata_link_resume() and
SCR access failure other than -EINVAL is considered an error condition
and exposes the incorrect return value bug as detection failure.  Fix
it.

Also, scsi_scr_cfg_write() was incorrectly returning 0 after it
ignored the request to write to SError.  Make it also return -EINVAL.

This was bisected and reported by Patrick McHardy.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>