append ht40- mode to config
use ht40+ if channel is lower than 5 on 2.4 ghz
force correct channel layout, if not override with channel 5
make number-checking more secure
Merge pull request #49 from FF-NRW/change-ht20-to-ht40

Change ht20 to ht40; force correct 2,4 GHz-Channel Layout