iwlwifi: clear scanning bits upon failure

In iwl_bg_request_scan function, if we could not send a
scan command it will go to done.
In done it does the right thing to call mac80211 with
scan complete, but the problem is STATUS_SCAN_HW is still
set causing any future scan to fail. Fix by clearing the scanning status
bits if scan fails.

Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>