net: l2: ieee802154: Add missing scan callback registration

Initializing an event callback with net_mgmt_init_event_callback() just
sets some of the callback fields but do not propagate those masks in the
core. If we want to use the callback, it is necessary to also call
net_mgmt_add_event_callback().

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>