net: ethernet: ti: cpsw: fix rx-usecs interrupt pacing consistency

The rx-usecs shouldn't be changed while interface down/up.
Currently, for instance, if it's set to 100us, after interface
down/up it's 500us. It's a hidden bug that can lead to lavish
interrupt pacing time increasing while "down/up" up to max value.

Steps to reproduce:
- set rx-usecs to be 100us
- down/up interface
- read new unexpected rx-usecs
Signed-off-by: NIvan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: NGrygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/ti/cpsw.c

@@ -1339,7 +1339,7 @@ static int cpsw_ndo_open(struct net_device *ndev)
 	if (priv->coal_intvl != 0) {
 		struct ethtool_coalesce coal;
 
-		coal.rx_coalesce_usecs = (priv->coal_intvl << 4);
+		coal.rx_coalesce_usecs = priv->coal_intvl;
 		cpsw_set_coalesce(ndev, &coal);
 	}