mac80211: wait for scan work complete before restarting hw

This is needed to avoid warning in ieee80211_restart_hw about hardware
scan in progress.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi W Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>