mac80211: fix rate reporting inside cfg80211_calculate_bitrate_he()

The reported rate is not scaled down correctly. After applying this patch,
the function will behave just like the v/ht equivalents.

Signed-off-by: Shashidhar Lakkavalli <slakkavalli@datto.com>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>