cfg80211: util: add 6GHz channel to freq conversion and vice versa

Extend the functions ieee80211_channel_to_frequency() and
ieee80211_frequency_to_channel() to support 6GHz band according
specification in 802.11ax D4.1 27.3.22.2.

Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Leon Zegers <leon.zegers@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Link: https://lore.kernel.org/r/1564745465-21234-4-git-send-email-arend.vanspriel@broadcom.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>