moxa: fix missing unlock on error in moxart_mac_start_xmit()

Add the missing unlock before return from function moxart_mac_start_xmit()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>