net/ieee802154: Fixing active scan buffer reference counting

Commit-ID 83ed3a29 changed the way
fragments are managed after being sent, and this brought a bug in the
active scan logic. Up to active scan to keep the buffer referenced
relevantly now, which is simpler than it used to be.

Reported-by: Johann Fischer <j.fischer@phytec.de>

Change-Id: I23db4bab878328b1ca3cb313e737de819177c281
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>