[libata] beautify module parameters

1. add defaults to description where possible
2. add value definition (off=0, on=1) where missing

v2: reformatted as per request by Jeff Garzik <jgarzik@pobox.com>
    "Enable foo (0=off, 1=on [default])"

Signed-off-by: Evgeni Golov <sargentd@die-welt.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>