moxa: fix the error handling in moxart_mac_probe()

This patch fix the error handling in moxart_mac_probe():
 - return -ENOMEM in some memory alloc fail cases
 - add missing free_netdev() in the error handling case

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