mac80211: update documentation about tx power

The structure member added at some point, but the kernel-doc was not
updated.

Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Link: https://lore.kernel.org/r/20200312144424.3023-1-hqjagain@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>