dpaa_eth: fix error return code in dpaa_eth_probe()

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

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>