iwlagn: refactor restart

The WoWLAN resume code will have to essentially
do a restart, but without going through the work
struct. To support that, refactor the restart by
splitting out the preparation code into a new
function.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>