net: pxa168_eth: remove unused variable 'retval' int pxa168_eth_change_mtu()

Fixes the following W=1 kernel build warning(s):

drivers/net/ethernet/marvell/pxa168_eth.c:1190:6: warning:
 variable 'retval' set but not used [-Wunused-but-set-variable]
 1190 |  int retval;
      |      ^~~~~~

Function pxa168_eth_change_mtu() always return zero, so variable 'retval'
is redundant, just remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>