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>
Git-commit: fa1f1085bc063da5a44f779c9b655b7026c52d68
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


Change-Id: I74d18fd05c7180ee8411334096392c5470c0d8ae
CRs-Fixed: 2536741
Signed-off-by: Liangwei Dong <liangwei@codeaurora.org>