net: dsa: fix fixed-link-phy device leaks

Make sure to drop the reference taken by of_phy_find_device() when
registering and deregistering the fixed-link PHY-device.

Fixes: 39b0c705 ("net: dsa: Allow configuration of CPU & DSA port
speeds/duplex")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>