net: fec_mxc: unregister mdio bus on probe error

If fecmxc_initialize_multi() fails, it frees but does not unregister
the mdio bus, causing subsequent uses of the "mii" command to crash.
Fix this by adding mdio_unregister() calls where needed.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Reviewed-by: Eric Nelson <eric@nelint.com>