netdevice: change net_device->promiscuity/allmulti to unsigned int

The comments of dev_set_allmulti/promiscuity() is that "While the count in
the device remains above zero...". So negative count is useless.
Fix the type of the counter from "int" to "unsigned int".

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>