iwlwifi: serialize station management actions

We are seeing some race conditions between incoming station management
requests (station add/remove) and the internal unassoc RXON command that
modifies station table. Modify these flows to require the mutex to be held
and thus serializing them.

This fixes http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=2207

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>