net: 802.15.4: Ignore too long network packets in dummy radio

If we receive too long message in dummy radio driver,
then we need to discard the packet as we cannot handle
it anyway.

Change-Id: Ibed0ac5f1568dd791dec3e780c7f6c71914d02cd
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>