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>

drivers/net/phy/Kconfig

@@ -119,8 +119,8 @@ config MICREL_PHY
 	  Supports the KSZ9021, VSC8201, KS8001 PHYs.
 
 config FIXED_PHY
-	bool "Driver for MDIO Bus/PHY emulation with fixed speed/link PHYs"
-	depends on PHYLIB=y
+	tristate "Driver for MDIO Bus/PHY emulation with fixed speed/link PHYs"
+	depends on PHYLIB
 	---help---
 	  Adds the platform "fixed" MDIO Bus to cover the boards that use
 	  PHYs that are not connected to the real MDIO bus.

drivers/net/phy/Makefile

@@ -17,7 +17,7 @@ obj-$(CONFIG_BCM87XX_PHY)	+= bcm87xx.o
 obj-$(CONFIG_ICPLUS_PHY)	+= icplus.o
 obj-$(CONFIG_REALTEK_PHY)	+= realtek.o
 obj-$(CONFIG_LSI_ET1011C_PHY)	+= et1011c.o
-obj-$(CONFIG_FIXED_PHY)		+= fixed.o
+obj-$(CONFIG_FIXED_PHY)		+= fixed_phy.o
 obj-$(CONFIG_MDIO_BITBANG)	+= mdio-bitbang.o
 obj-$(CONFIG_MDIO_GPIO)		+= mdio-gpio.o
 obj-$(CONFIG_NATIONAL_PHY)	+= national.o

include/linux/phy_fixed.h

@@ -11,7 +11,7 @@ struct fixed_phy_status {
 
 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,