Fix x-prop issue in ScratchpadSlavePort (#2818)

* Fix x-prop issue in ScratchpadSlavePort

Don't let D$ req.cmd field become X, or else it'll make req.ready become X,
which confuses the Rocket core.

* Fix regression introduced in fix

Wrong D$ command was sent on first slave-port access.

Co-authored-by: Andrew Waterman <andrew@sifive.com>