nl80211: Add option to adjust transmit power

This patch adds transmit power setting type and transmit power level attributes
to NL80211_CMD_SET_WIPHY in order to facilitate adjusting of the transmit power
level of the device.

The added attributes allow selection of automatic, limited or fixed transmit
power level, with the level definable in signed mBm format.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>