net: l2: Add enable callback

This adds enable callback which can be used to notify the L2 driver about
changes of interface state, the L2 driver can then check if the new state
is allowed and reject otherwise.

Change-Id: I4bb6b1e32be2633f24694c0246585f803f8c645d
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>