net: phy: rework Kconfig settings for MDIO_BUS

I still see build errors in randconfig builds and have had this
patch for a while to locally work around it:

drivers/built-in.o: In function `xgene_mdio_probe':
mux-core.c:(.text+0x352154): undefined reference to `of_mdiobus_register'
mux-core.c:(.text+0x352168): undefined reference to `mdiobus_free'
mux-core.c:(.text+0x3521c0): undefined reference to `mdiobus_alloc_size'

The idea is that CONFIG_MDIO_BUS now reflects whether the mdio_bus
code is built-in or a module, and other drivers that use the core
code can simply depend on that, instead of having a complex
dependency line.

Fixes: 90eff909 ("net: phy: Allow splitting MDIO bus/device support from PHYs")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/Kconfig

@@ -7,7 +7,16 @@ menuconfig MDIO_DEVICE
 	help
 	   MDIO devices and driver infrastructure code.
 
-if MDIO_DEVICE
+config MDIO_BUS
+	tristate
+	default m if PHYLIB=m
+	default MDIO_DEVICE
+	help
+	  This internal symbol is used for link time dependencies and it
+	  reflects whether the mdio_bus/mdio_device code is built as a
+	  loadable module or built-in.
+
+if MDIO_BUS
 
 config MDIO_BCM_IPROC
 	tristate "Broadcom iProc MDIO bus controller"
@@ -28,7 +37,6 @@ config MDIO_BCM_UNIMAC
 
 config MDIO_BITBANG
 	tristate "Bitbanged MDIO buses"
-	depends on !(MDIO_DEVICE=y && PHYLIB=m)
 	help
 	  This module implements the MDIO bus protocol in software,
 	  for use by low level drivers that export the ability to
@@ -127,7 +135,6 @@ config MDIO_THUNDER
 	tristate "ThunderX SOCs MDIO buses"
 	depends on 64BIT
 	depends on PCI
-	depends on !(MDIO_DEVICE=y && PHYLIB=m)
 	select MDIO_CAVIUM
 	help
 	  This driver supports the MDIO interfaces found on Cavium