net: dsa: slave: fix of-node leak and phy priority

Make sure to drop the reference taken by of_parse_phandle() before
returning from dsa_slave_phy_setup().

Note that this also modifies the PHY priority so that any fixed-link
node is only parsed when no phy-handle is given, which is in accordance
with the common scheme for this.

Fixes: 0d8bcdd3 ("net: dsa: allow for more complex PHY setups")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>