net: enetc: automatically select IERB module

Now that enetc supports flow control we have to make sure the settings in
the IERB are correct. Therefore, we actually depend on the enetc-ierb
module. Previously it was possible that this module was disabled while the
enetc was enabled. Fix it by automatically select the enetc-ierb module.

Fixes: e7d48e5f ("net: enetc: add a mini driver for the Integrated Endpoint Register Block")
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>