net: dsa: Add error path handling in dsa_tree_setup()

In case a call to dsa_tree_setup() fails, an attempt to cleanup is made
by calling dsa_tree_remove_switch(), which should take care of
removing/unregistering any resources previously allocated. This does not
happen because it is conditioned by dst->setup being true, which is set
only after _all_ setup steps were performed successfully.

This is especially interesting when the internal MDIO bus is registered
but afterwards, a port setup fails and the mdiobus_unregister() is never
called. This leads to a BUG_ON() complaining about the fact that it's
trying to free an MDIO bus that's still registered.

Add proper error handling in all functions branching from
dsa_tree_setup().

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reported-by: kernel test robot <rong.a.chen@intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>