iwlwifi: trigger scan synchronously

Scan requesting doesn't need to be asynchronous
since all code paths leading up to it can sleep.
Make the scan request a new util operation that
is hw-specific (to account for 3945 vs. agn)
and call it right in place.

This patch moves a lot of code into iwlagn as
it need not be in iwlcore.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>