libata: Don't disable dipm with SET FEATURES

LPM seems to get hung up while disabling DIPM, and after thinking
about this a bit, I don't think we really need to manually disable it
anyway.

Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>