net: macb: fix missing unlock on error in macb_start_xmit()

Fix missing unlock before return from function macb_start_xmit()
in the error handling case.

Fixes: 007e4ba3 ("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>