net: Allow FIXED_PHY to be modular.

Otherwise we get things like:

warning: (NET_DSA_BCM_SF2 && BCMGENET && SYSTEMPORT) selects FIXED_PHY which has unmet direct dependencies (NETDEVICES && PHYLIB=y)

In order to make this work we have to rename fixed.c to fixed_phy.c
because the regulator drivers already have a module named "fixed.o".

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/phy_fixed.h b/include/linux/phy_fixed.h
index f2ca1b4..7e75bfe 100644
--- a/include/linux/phy_fixed.h
+++ b/include/linux/phy_fixed.h
@@ -11,7 +11,7 @@
 
 struct device_node;
 
-#ifdef CONFIG_FIXED_PHY
+#if IS_ENABLED(CONFIG_FIXED_PHY)
 extern int fixed_phy_add(unsigned int irq, int phy_id,
 			 struct fixed_phy_status *status);
 extern struct phy_device *fixed_phy_register(unsigned int irq,