net: l2: ieee802154: mgmt: allow beacon payload

The standard does allow for a optional beacon payload, which gets lost
during scan, that could be interesting for the application to access
in the NET_EVENT_IEEE802154_SCAN_RESULT callback.

See section 7.3.1.6 in IEEE Std 802.15.4 for more information about
the beacon payload field. And section 7.3.1 and figure 7-5 about general
beacon frame format.

Signed-off-by: Fabian Pflug <fabian.pflug@grandcentrix.net>