nl80211: read TX power using NL80211_CMD_GET_INTERFACE

With the mac80211 commit d55d0d598e66 ("nl80211: put current TX power in
interface info") it is possible now to get TX power using nl80211. As we
don't really support any wext-only drivers it doesn't make sense to
leave wext as a fallback.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>