net: fec: Only unregister MII bus if we registered it

If we fail to probe for whatever reason, we cannot unregister/free the
MII bus unless we registered it with fec_get_miibus. This fixes FECs
sharing an MDIO bus from destroying it, preventing the other FEC from
using it.

Fixes: 6a895d03 ("net: Update eQos driver and FEC driver to use eth phy interfaces")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>