net: l2: ieee802154: readability: buf -> pkt_buf

This change renames buf to pkt_buf in one very specific instance to
distinguish the packet buffer from the frame buffer which is kept in the
same local scope. In all other instances the meaning of "buf" should be
obvious from context, not so here, though

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