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

Fixes: 712c3185 ("net: thunderx: Program LMAC credits based on MTU")
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>