net: dsa: Fix functional dsa-loop dependency on FIXED_PHY

We have a functional dependency on the FIXED_PHY MDIO bus because we register
fixed PHY devices "the old way" which only works if the code that does this has
had a chance to run before the fixed MDIO bus is probed. Make sure we account
for that and have dsa_loop_bdinfo.o be either built-in or modular depending on
whether CONFIG_FIXED_PHY reflects that too.

Fixes: 98cd1552 ("net: dsa: Mock-up driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>