net: l2: ieee802154: document endianness

The IEEE 802.15.4 L2 code stores representation of attributes like
PAN id, short address and extended address in different encodings:

* big endian for extended address and CPU byte order for everything
  else whenever such attributes enter user space (except for IP/socket
  link layer addresses which are always big endian - even in case of
  short addresses - to maintain POSIX compatibility).

* little endian for everything that is close to the radio driver as
  IEEE 802.15.4 frames are little endian encoded.

Endianness was almost nowhere documented which led to several bugs and
inconsistencies where assignments of different byte order were not
converted (or sometimes converted, sometimes not).

This change documents endianness wherever possible within the realm of
the IEEE 802.15.4 L2 code. Conversion bugs and inconsistencies that were
revealed by the improved documentation will be fixed in a separate
commit.

Signed-off-by: Florian Grandel <jerico.dev@gmail.com>