cfg80211: Adjust 6 GHz frequency to channel conversion

Adjust the 6 GHz frequency to channel conversion function,
the other way around was previously handled.

Signed-off-by: Amar Singhal <asinghal@codeaurora.org>
Link: https://lore.kernel.org/r/1592599921-10607-1-git-send-email-asinghal@codeaurora.org
[rewrite commit message, hard-code channel 2]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>