net: cpmac: fix error handling of cpmac_probe()

Add the missing free_netdev() before return from function
cpmac_probe() in the error handling case.
This patch revert commit 0465be8f

 ("net: cpmac: fix in
releasing resources"), which changed to only free_netdev
while register_netdev failed.

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