net: ieee802154: Expose basic network parameters through net mgmt API

It is then possible to set the channel, pan_id and short address through
this API. Such features are mainly useful for testing purposes.

Change-Id: I41aeb397afdb231458a3b13638f3e13d3ac28a6c
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>