cxgbe(4): Fix inverted test in sysctl_loadavg

Only internal debug firmwares report the μP load averages so this bug
did not affect regular users.

Fixes:	e19d84979a cxgbe(4): Block most access to the hardware as soon as the adapter stops.
MFC after:	1 week
Sponsored by:	Chelsio Communications

sys/dev/cxgbe/t4_main.c

@@ -9016,7 +9016,7 @@ sysctl_loadavg(SYSCTL_HANDLER_ARGS)
 	rc = begin_synchronized_op(sc, NULL, SLEEP_OK | INTR_OK, "t4lavg");
 	if (rc)
 		return (rc);
-	if (hw_all_ok(sc))
+	if (!hw_all_ok(sc))
 		rc = ENXIO;
 	else {
 		param = V_FW_PARAMS_MNEM(FW_PARAMS_MNEM_DEV) |