mac80211_hwsim: use per-interface power level

When channel contexts are used, there's no global power level
(the power_level is always 0). Use the per-interface TX power
in mac80211_hwsim to have a proper setting for both cases.

This fixes the bgscan_simple and bgscan_learn test cases when
the number of channels advertised by hwsim is >1 by default.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>