iwlwifi: cleanup station adding code

The work done when a station is added is very similar whether the station
is added synchronously or asynchronously. Centralize this work. At the same
time increase the status flags being checked for when the command returns
with accompanying debug messages. Also increase checking when setting the
"ucode active" state with accompanying debugging.

This work is done in preparation for station notification support.

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