net: make net sysfs attributes ro_after_init

The attributes of net devices are immutable.

Ideally, attribute groups would contain const attributes
but there are too many places that do modifications of list
during startup (in other code) to allow that.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>