[SCSI] libsas: fix scr_read/write users and update the libata documentation

This fixes up the usage in libsas (which are easy to miss, since they're
only in the scsi-misc tree) ... and also corrects the documentation on
the point of what these two function pointers actually return.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>