net: fec: Fix multicast list setup in fec_restart().

Setup the multicast list of the net_device instead of
clearing it blindly. This restores the multicast groups
in case of a link down/up event or when resuming from
suspend.

Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Signed-off-by: David S. Miller <davem@davemloft.net>