[libata] Slightly improved no-op REQUEST SENSE, SEND DIAGNOSTIC

A few pedantic apps care about missing or lame "mandatory" SCSI
commands, so

REQUEST SENSE -- as we autosense, R.S. just returns zeroes

SEND DIAGNOSTIC -- our default (no-op) self-test succeeds, all
		   other requests for testing fail.

Signed-off-by: Jeff Garzik <jeff@garzik.org>