sata_sil24: replace sil24_update_tf() with sil24_read_tf()

Replace sil24_update_tf() to sil24_read_tf() which reads TF into
passed int result TF argument and can read TFs of PMP links.  This
will be used by PMP support.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>