luci-mod-status: channel_analysis: detect 40 MHz (20+20 bonded) APs

luci-mod-status: channel_analysis: detect 40 MHz (20+20 bonded) APs

At the moment, 40 MHz detection works only for some APs. HT (802.11n)
exposes the 40 MHz capability of APs in two ways. The first way is
a continuous 40 Mhz band. The second way is two 20MHz bonded channels.
Linux detects the presence of 40 MHz by checking the presence of the
second channel. It is always absent when the AP supports only 20 MHz.
This PR fixes this issue and both ways are supported.

First (40 MHz continuous):
HT Operation:
     Primary Channel: 13
     Secondary Channel Offset: below
     Channel Width: 40 MHz or higher

Second way (20+20MHz bonded):
HT Operation:
     Primary Channel: 1
     Secondary Channel Offset: above
     Channel Width: 20 MHz

Pure 20MHz channel:
HT Operation:
     Primary Channel: 1
     Secondary Channel Offset: no secondary
     Channel Width: 20 MHz

Fixes: #6839
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>