libertas: Improvements on automatic tx power control via SIOCSIWTXPOW.

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>