net: l2: ieee802154: expose implicit frame assumption

The current IEEE 802.15.4 stack assumes that drivers will
not provide more than one fragment in incoming packages.

This change exposes and enforces the pre-existing assumption
made by the implementation.

So far this is not a limiting restriction as MPDUs with more
than 127 bytes are not supported yet. It will probably have to
be changed as soon as larger payloads (as allowed by PHYs
introduced more recently into the spec) might want to allocate
smaller fragments.

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