net: mgmt: Use layer codes defined in net_mgmt.h

Instead of hard coding some random value to layer code, use the
code that is registered in net_mgmt.h
This way it is easier to keep track of used layer codes in the
future, and we also allow out-of-tree components a way to avoid
allocating same layer code to in-tree-source code.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>