cfg80211/mac80211: Update set_tx_power to use mBm instead of dBm units

In preparation for a TX power setting interface in the nl80211, change the
.set_tx_power function to use mBm units instead of dBm for greater accuracy and
smaller power levels.

Also, already in advance move the tx_power_setting enumeration to nl80211.

This change affects the .tx_set_power function prototype. As a result, the
corresponding changes are needed to modules using it. These are mac80211,
iwmc3200wifi and rndis_wlan.

Cc: Samuel Ortiz <samuel.ortiz@intel.com>
Cc: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Acked-by: Samuel Ortiz <samuel.ortiz@intel.com>
Acked-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>