net: l2: ieee802154: mgmt: NET_CONTINUE unless pkt is unrefed

It is the general rule in Zephyr's network stack that methods that
return NET_OK are expected to have "consumed" the packet, i.e. it should
not be referenced any more.

This change applies this rule to the methods in ieee802154_mgmt.* for
improved consistency with the remainder of the network stack.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>