mac80211_hwsim: Use proper TX power

In struct ieee80211_tx_info, control.vif pointer and rate_driver_data[0]
falls on the same place, depending on the union usage.
During the whole TX process, the union is referred to as a control struct,
which holds the vif that is later used in the tx flow, especially in order
to derive the used tx power.
Referring direcly to rate_driver_data[0] and assigning a value to it,
overwrites the vif pointer, hence making all later references irrelevant.
Moreover, rate_driver_data[0] isn't used later in the flow in order to
retrieve the channel that it is pointing to.

Cc: stable@vger.kernel.org
Signed-off-by: Beni Lev <beni.lev@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>