nl80211: Report max TX power in NL80211_BAND_ATTR_FREQS

This is useful information to provide for userspace (e.g., hostapd needs
this to generate Country IE).

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>