Fix missing unlock before return from function macb_start_xmit()
in the error handling case.
Fixes: 007e4ba3ee13 ("net: macb: initialize checksum when using
checksum offloading") Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com> Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com> Signed-off-by: David S. Miller <davem@davemloft.net>