[netdrvr] Drivers should not set IFF_* flag themselves

Some hardware set promisc when they are requested to set IFF_ALLMULTI flag.
It's ok, but if drivers set IFF_PROMISC flag when they set promisc,
it will broken upper layer handle for promisc and allmulti.
In addition, drivers can use their own hardware programming to make it.
So do not allow drivers to set IFF_* flags.

This is a general driver fix, so I didn't split it to pieces and send
to specific driver maintainers.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>