mac80211: cancel scan in ieee80211_restart_hw if software scan pending

This function exists to clean-up after a hardware error or something
similar.  The restart is accomplished using the same infrastructure used
to resume after a suspend.  The suspend path cancels running scans, so
it seems appropriate to do that here as well for software-based scans.
If a hardware-based scan is pending, issue a warning message since this
indicates that the drivers has failed to clean-up after itself.

Signed-off-by: John W. Linville <linville@tuxdriver.com>