net/ieee802154: Security session must be initialized with a valid key

Security layer was tested only against CC2520, which does not mandate to
begin the session with a valid key. However, the crypto API tells it
must do so. And indeed, starting a session on mtls shim crypto device
will fail due to missing key. Thus moving the relevant code where it
should.

Reported-by: Johann Fischer <j.fischer@phytec.de>

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