cpmac: do not leak struct net_device on phy_connect errors

If the call to phy_connect fails, we will return directly instead of freeing
the previously allocated struct net_device.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
CC: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>