[MACVLAN]: Setting macvlan_handle_frame_hook to NULL when rtnl_link_register() fails.

In drivers/net/macvlan.c, when rtnl_link_register() fails in
macvlan_init_module(), there is no point to set it (second time in
this method) to macvlan_handle_frame; macvlan_init_module() will
return a negative number, so instead this patch sets
macvlan_handle_frame_hook to NULL.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>