ibm_newemac: Don't call dev_mc_add() before device is registered

We must not call dev_mc_add() from within our HW configure which happens
before we initialize and register the netdev. Do it in open() instead.

Thanks to Sebastian Siewior for tracking it down.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>