net80211: provide a set of ieee80211_add_channel*() functions

This change 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 -
they will allow to hide implementation details).

• ieee80211_add_channel_list_2ghz()
• ieee80211_add_channel_list_5ghz()

(mostly as a replacement for ieee80211_init_channels() - they will allow
to specify non-default channel list; may be used in ic_getradiocaps()).

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

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D6124