mac80211: ask driver to look at power level when starting AP

The power level might have been set, but as the interface was idle
it might not have taken effect yet. Ask the driver to check the
power level when starting up an AP so that in this case the correct
power level is used in case the device/driver can only set it when
the interface is actually active.

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