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>