mac80211: initialize power mode for mesh STAs

Previously the default mesh STA nonpeer power mode was
UNKNOWN (0) make the default mesh STA power mode ACTIVE,
to prevent unnecessary  frame buffering while peering is
not yet complete. Fixes a panic in ath9k_htc when adding
stations from userspace, and mcast buffered frames are
later released.

Thanks to Bob Copeland for his help debugging this.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>