MFC r343340:
net80211: fix channel list construction for non-auto operating mode.

Change the way how channel list mode <-> desired mode match is done:

• Match channel list mode for next non-auto desired modes:
  • 11b: 11g, 11ng;
  • 11a: 11na
• Add pre-defined channels only when one of the next conditions met:
  • the desired channel mode is 'auto' or
  • the desired channel and selected channel list modes are exactly

the same or

• the previous rule (11g / 11n promotion) applies.

Before r275875 construction work properly for all except
11ng / 11na modes - these were broken at all
(i.e., the scan list was empty); after r275875 all checks were removed,
so scan table was populated by all device-compatible channels
(desired mode was ignored).

For example, if I will set 'ifconfig wlan0 mode 11ng' for RTL8821AU:

• pre-r275875: nothing, scan will not work;
• after r275875: both 11ng and 11na bands were scanned; also, since 11b

channel list was used, 14th channel was scanned too.

• after this change: only 11ng - 1-13 channels - are used for scanning.

Note: since 11-stable does not have VHT mode definitions
they were removed from this merge.