kernel: fix kmod-ieee802154_6lowpan config symbol

Fixes build failure caught by buildbot:

IEEE Std 802.15.4 Low-Rate Wireless Personal Area Networks support (IEEE802154) [M/n/y/?] m
  6lowpan support over IEEE 802.15.4 (IEEE802154_6LOWPAN) [N/m/?] (NEW) aborted!

Console input/output is redirected. Run 'make oldconfig' to update configuration.

make[6]: *** [silentoldconfig] Error 1
make[5]: *** [silentoldconfig] Error 2

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45357 3c298f89-4303-0410-b956-a3cf2f4a3e73