net: Increase the 802.15.4 TX stack to 4096 bytes

If application needs to send large packets, the 802.15.4
fragmentation code needs to store the fragments somewhere.
Currently this somewhere is stack which means that we
need to increase the stack size quite a lot.

Change-Id: I9b08563e77c021e5ac103e637e331d7b977563cc
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>