ieee802154: change _cb handling slightly

The current mac_cb handling of ieee802154 is rather awkward and limited.
Decompose the single flags fields into multiple fields with the meanings
of each subfield of the flags field to make future extensions (for
example, link-layer security) easier. Also don't set the frame sequence
number in upper layers, since that's a thing the MAC is supposed to set
on frame transmit.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>