net: fix dev_set_promiscuity() breakage

Commit dad9b335 (netdevice: Fix promiscuity and allmulti overflow) broke
dev_set_promiscuity() by returning on success without reprogramming the
device.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>