net: l2: ieee802154: decouple frame decryption from upper layer fields

The L2 function `ieee802154_decipher_data_frame()` relied on upper layer
LL address fields which breaks encapsulation.

Also fixes a bug introduced in another fix that went overboard (#53734).

Fixes: #78490

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