powerpc/ep8248e: Fix compile problem if !CONFIG_FS_ENET

If we don't enable FS_ENET we get build issues:

arch/powerpc/platforms/built-in.o: In function `ep8248e_mdio_probe':
arch/powerpc/platforms/82xx/ep8248e.c:129: undefined reference to `alloc_mdio_bitbang'
arch/powerpc/platforms/82xx/ep8248e.c:143: undefined reference to `mdiobus_register'

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>