Wext makes no assumptions about the contents of
data->txpower.fixed and data->txpower.value when
data->txpower.disabled is set, so do not update
the user-requested power level while disabling.
Also, when wext configures a really _fixed_ power
output [1], we should reject it instead of limiting it
to the regulatory constraint. If the user wants to set
a _limit_ [2] then we should honour that.