ieee80211: add definition for transmit power envelope element

IEEE Std 802.11ax™-2021 makes changes to the transmit power envelope
element, adjust the code accordingly.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Link: https://lore.kernel.org/r/20210820122041.12157-7-wgong@codeaurora.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>