drivers: ieee802154: nrf: wrapped pkt attribute access

Nordic's IEEE 802.15.4 radio driver adapter layer had a few raw accesses
to net packet attributes.

Packet attributes should never be accessed directly, though, but only
through the dedicated accessor methods provided by the net core.

This change replaces raw accesses to packet attributes by their
respective wrapper functions.

This also is a necessary precondition to the isolation and
encapsulation of IEEE 802.15.4 specific packet attributes which will be
introduced in a later commit of this change set.

Fix follows.