net: dsa: mv88e6xxx: Fix clearing of stats counters

The mv88e6161 would sometime fail to probe with a timeout waiting for
the switch to complete an operation. This operation is supposed to
clear the statistics counters. However, due to a read/modify/write,
without the needed mask, the operation actually carried out was more
random, with invalid parameters, resulting in the switch not
responding. We need to preserve the histogram mode bits, so apply a
mask to keep them.
Reported-by: Chris Healy <Chris.Healy@zii.aero>
Fixes: 40cff8fc ("net: dsa: mv88e6xxx: Fix stats histogram mode")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/global1.c

@@ -567,6 +567,8 @@ int mv88e6xxx_g1_stats_clear(struct mv88e6xxx_chip *chip)
 	if (err)
 		return err;
 
+	/* Keep the histogram mode bits */
+	val &= MV88E6XXX_G1_STATS_OP_HIST_RX_TX;
 	val |= MV88E6XXX_G1_STATS_OP_BUSY | MV88E6XXX_G1_STATS_OP_FLUSH_ALL;
 
 	err = mv88e6xxx_g1_write(chip, MV88E6XXX_G1_STATS_OP, val);