net: l2: ieee802154: enc-only security level removed from spec

The encryption-only security level has been removed from the
spec, see IEEE 802.15.4-2020, 9.4.2.2 Security Level field.

The standard provides the following explanation (ibid):

"This security level is deprecated and shall not be used in
implementation compliant with this standard. Devices that
receive frames with security level 4 shall discard them, as
described in 9.2.4. The counter mode encryption and cipher
block chaining message authentication code (CCM) used allows
trivial changes to the underlaying encrypted data unless
data authenticity is provided, thus using data confidentiality
only is not useful. In the case of TSCH mode, security level 4
allows higher security level frames to be downgraded to
security level 4 frames."

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