net: macvlan: remove redundant initialization in macvlan_dev_netpoll_setup

The initialization for err with 0 seems useless, as it is soon updated
with -ENOMEM. So, we can remove it.

Changes since v1:
-Keep -ENOMEM still.

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Link: https://lore.kernel.org/r/1604541244-3241-1-git-send-email-dong.menglong@zte.com.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>