net: dsa: Unregister slave_dev in error path

With commit 0071f56e ("dsa: Register netdev before phy"), we are now trying
to free a network device that has been previously registered, and in case of
errors, this will make us hit the BUG_ON(dev->reg_state != NETREG_UNREGISTERED)
condition.

Fix this by adding a missing unregister_netdev() before free_netdev().

Fixes: 0071f56e

 ("dsa: Register netdev before phy")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>