net: enetc: fix link error again

A link time bug that I had fixed before has come back now that
another sub-module was added to the enetc driver:

ERROR: modpost: "enetc_ierb_register_pf" [drivers/net/ethernet/freescale/enetc/fsl-enetc.ko] undefined!

The problem is that the enetc Makefile is not actually used for
the ierb module if that is the only built-in driver in there
and everything else is a loadable module.

Fix it by always entering the directory this time, regardless
of which symbols are configured. This should reliably fix the
problem and prevent it from coming back another time.

Fixes: 112463dd ("net: dsa: felix: fix link error")
Fixes: e7d48e5f

 ("net: enetc: add a mini driver for the Integrated Endpoint Register Block")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>