net: enetc: fix MAC Merge layer remaining enabled until a link down event

Current enetc_set_mm() is designed to set the priv->active_offloads bit
ENETC_F_QBU for enetc_mm_link_state_update() to act on, but if the link
is already up, it modifies the ENETC_MMCSR_ME ("Merge Enable") bit
directly.

The problem is that it only *sets* ENETC_MMCSR_ME if the link is up, it
doesn't *clear* it if needed. So subsequent enetc_get_mm() calls still
see tx-enabled as true, up until a link down event, which is when
enetc_mm_link_state_update() will get called.

This is not a functional issue as far as I can assess. It has only come
up because I'd like to uphold a simple API rule in core ethtool code:
the pMAC cannot be disabled if TX is going to be enabled. Currently,
the fact that TX remains enabled for longer than expected (after the
enetc_set_mm() call that disables it) is going to violate that rule,
which is how it was caught.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/freescale/enetc/enetc_ethtool.c

@@ -1041,10 +1041,13 @@ static int enetc_set_mm(struct net_device *ndev, struct ethtool_mm_cfg *cfg,
 	else
 		priv->active_offloads &= ~ENETC_F_QBU;
 
-	/* If link is up, enable MAC Merge right away */
-	if (!!(priv->active_offloads & ENETC_F_QBU) &&
-	    !(val & ENETC_MMCSR_LINK_FAIL))
-		val |= ENETC_MMCSR_ME;
+	/* If link is up, enable/disable MAC Merge right away */
+	if (!(val & ENETC_MMCSR_LINK_FAIL)) {
+		if (!!(priv->active_offloads & ENETC_F_QBU))
+			val |= ENETC_MMCSR_ME;
+		else
+			val &= ~ENETC_MMCSR_ME;
+	}
 
 	val &= ~ENETC_MMCSR_VT_MASK;
 	val |= ENETC_MMCSR_VT(cfg->verify_time);