iwlwifi: don't throw error when trying to remove IGTK

The IGTK keys are only removed by mac80211 after it has already
removed the AP station.  This causes the driver to throw an error
because mac80211 is trying to remove the IGTK when the station doesn't
exist anymore.

The firmware is aware that the station has been removed and can deal
with it the next time we try to add an IGTK for a station, so we
shouldn't try to remove the key if the station ID is
IWL_MVM_INVALID_STA.  Do this by removing the check for mvm_sta before
calling iwl_mvm_send_sta_igtk() and check return from that function
gracefully if the station ID is invalid.

Cc: stable@vger.kernel.org # 4.12+
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>