net: phy: move config symbol MDIO_BUS to drivers/net/phy/Kconfig

Config symbol MDIO_BUS isn't used in drivers/net/mdio. It's only used
in drivers/net/phy. So move it there.

Signed-off-by: Heiner Kallweit <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/mdio/Kconfig

···
 # MDIO Layer Configuration
 #
 
-config MDIO_BUS
-	tristate "MDIO bus consumer layer"
-	help
-	  MDIO bus consumer layer
-
 if PHYLIB
 
 config FWNODE_MDIO

drivers/net/phy/Kconfig

···
 # PHY Layer Configuration
 #
 
+config MDIO_BUS
+	tristate "MDIO bus consumer layer"
+	help
+	  MDIO bus consumer layer
+
 config PHYLINK
 	tristate
 	select PHYLIB