net: mvneta: remove redundant check for eee->tx_lpi_timer < 0

fixes the smatch warning:

drivers/net/ethernet/marvell/mvneta.c:4252 mvneta_ethtool_set_eee() warn:
 unsigned 'eee->tx_lpi_timer' is never less than zero.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mvneta.c

@@ -4248,8 +4248,7 @@ static int mvneta_ethtool_set_eee(struct net_device *dev,
 
 	/* The Armada 37x documents do not give limits for this other than
 	 * it being an 8-bit register. */
-	if (eee->tx_lpi_enabled &&
-	    (eee->tx_lpi_timer < 0 || eee->tx_lpi_timer > 255))
+	if (eee->tx_lpi_enabled && eee->tx_lpi_timer > 255)
 		return -EINVAL;
 
 	lpi_ctl0 = mvreg_read(pp, MVNETA_LPI_CTRL_0);