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>

drivers/net/dsa/Makefile

 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_NET_DSA_BCM_SF2)	+= bcm-sf2.o
 bcm-sf2-objs			:= bcm_sf2.o bcm_sf2_cfp.o
-obj-$(CONFIG_NET_DSA_LOOP)	+= dsa_loop.o dsa_loop_bdinfo.o
+obj-$(CONFIG_NET_DSA_LOOP)	+= dsa_loop.o
+ifdef CONFIG_NET_DSA_LOOP
+obj-$(CONFIG_FIXED_PHY)		+= dsa_loop_bdinfo.o
+endif
 obj-$(CONFIG_NET_DSA_MT7530)	+= mt7530.o
 obj-$(CONFIG_NET_DSA_MV88E6060) += mv88e6060.o
 obj-$(CONFIG_NET_DSA_QCA8K)	+= qca8k.o