Fix to return a negative error code from the cpsw_fill_rx_channels()
error handling case instead of 0, as done elsewhere in this function.

Fixes: ce52c744 ("net: ethernet: ti: cpsw: add ethtool channels support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>