net: l2: ieee802154: readability: LL header length

Several distinct naming conventions existed within the IEEE 802.15.4
stack wrt header length. This change converges to a single naming
convention, the one that is less ambiguous and already most used.

The change also makes the distinction between L2 (link layer/LL) header
length and 6LoWPAN fragmentation header length to avoid confusion.

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