iwlwifi: mvm: Change number of associated stations when station becomes associated

Currently, the number of associated stations gets updated when adding
a new station or removing it. This is incorrect as it's possible that
a station was inserted before it was associated
Fix this by increasing/decreasing ap_assoc_sta_count whenever
a station transitions in/out the associated state.

Signed-off-by: Ayala Beker <ayala.beker@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>