sata_mv ncq Restrict max sectors to 8-bits on GenII NCQ

The GenII chips have only 8-bits for the sector_count field when performing NCQ.
Add a dev_config method to restrict this when necessary, taking care not to
override any other restriction already in place (likely none, but someday.. ?).

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>