net: ieee802154: Fix current stack to completely drop CRC handling

The 802.15.4 frame has a header and in the end after the payload a CRC.
Overall MTU is 127 bytes, but CRC is most likely going to be generated
by the device itself (offloading). Because the limitation of current
nbuf, it's not possible to keep this CRC in the buffer because it
require a user data size of 127 bytes but if we do so, IP stack will
fill in as much as it can as packet data, thus occupying the 2 last
bytes. This generates bugs. A perfect solution would be to generalize a
better handling of MTU and head or tail reserve data into nbuf, but
this will probably be solved in the future.

Change-Id: I1a0fee4d555e2717c1edd5afba399a1f17d9c7a9
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>