net: dsa: felix: fix link error

When the enetc driver is disabled, the mdio support fails to
get built:

drivers/net/dsa/ocelot/felix_vsc9959.o: In function `vsc9959_mdio_bus_alloc':
felix_vsc9959.c:(.text+0x19c): undefined reference to `enetc_hw_alloc'
felix_vsc9959.c:(.text+0x1d1): undefined reference to `enetc_mdio_read'
felix_vsc9959.c:(.text+0x1d8): undefined reference to `enetc_mdio_write'

Change the Makefile to enter the subdirectory for this as well.

Fixes: bdeced75 ("net: dsa: felix: Add PCS operations for PHYLINK")
Fixes: 6517798d ("enetc: Make MDIO accessors more generic and export to include/linux/fsl")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>