amd-xgbe: use dma_mapping_error to check map errors

The dma_mapping_error() returns true or false, but we want
to return -ENOMEM if there was an error.

Fixes: 174fd259 ("amd-xgbe: Implement split header receive support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>