nl80211: put current TX power in interface info

Many drivers implement reading current TX power (using either cfg80211
or ieee80211 op) but userspace can't get it using nl80211. Right now the
only way to access it is to call some wext ioctl.
Let's put TX power in interface info reply (callback is wdev specific)
just like we do with current channel.
To be consistent (e.g. NL80211_CMD_SET_WIPHY) let's use mBm as na unit.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>