net: l2: ieee802154: Fix packed structure warning

`struct ieee802154_mpdu` access generates warnings related to unaligned
access to packed structure member. The structure itself however does not
need to be packed, since it is not mapped directly into the frame
buffer, but rather contains pointers to the structures representing
corresponding header regions in the frame. Those structures are
correctly defined as packed.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>