iwlwifi: mvm: Fix removal of IGTK

When removing an IGTK, iwl_mvm_send_sta_igtk() was
called before station ID was retrieved, so the function
was invoked with an invalid station ID. Fix this by first
getting the station ID.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=192411


Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>