iwlwifi: don't fail if scan is issued too early

This patch returns success and empty scan on scans requests that were
rejected because issued too early. The cached bss list from previous
scanning will be returned by mac80211.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>