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>