net80211: provide a set of ieee80211_add_channel*() functions

[avos]

Summary

This diff adds few methods for net80211 channel table setup:

• ieee80211_add_channel()
• ieee80211_add_channel_ht40()

(primarily for drivers, that parse EEPROM to get channel list)

• ieee80211_add_channel_list_2ghz()
• ieee80211_add_channel_list_5ghz()

(mostly as a replacement for ieee80211_init_channels(); may be used in ic_getradiocaps())

TODO: reduce number of parameters (6 - 8 at this time).

Test Plan

Tested with wpi(4) (add_channel) and rum(4) (add_channel_list_2ghz).

[adrian]

ok, get some more testing of this (including ath(4), which does bad things to channel setup, iirc) and then i'll okay it. It looks fine as a first pass though, so thanks!

And this refactor also makes it easier to add VHT20/40/80 channel support when it comes time to do that.