net: stmmac: dwmac-oxnas: fix fixed-link-phydev leaks

Make sure to deregister and free any fixed-link phy registered during
probe on probe errors and on driver unbind by calling the new glue
helper function.

For driver unbind, use the generic stmmac-platform remove implementation
and add an exit callback to disable the clock.

Fixes: 5ed74140 ("net: stmmac: Add OXNAS Glue Driver")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>