iwconfig txpower can now be used to set tx power to fixed or auto. If set to
auto the default firmware settings are used.

The command CMD_802_11_PA_CFG is only sent to older firmware, as Dan Williams
noted the command was no longer supported in firmware V9+.

Signed-off-by: Anna Neal <anna@cozybit.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>