[libata] sata_mv: more S/G fixes

* corruption fix: we only want the lower 16 bits of length (0 == 64kb)

* ditto: the upper layer sets max-phys-segments to LIBATA_MAX_PRD,
  so we must reset it to own hw-specific length.

* delete unused mv_fill_sg() return value

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>