iwlwifi: mvm: document switch case fall-through in iwl_mvm_send_sta_key

Add a comment indicating that the WLAN_CIPHER_SUITE_WEP104 case falls
through to the WLAN_CIPHER_SUITE_WEP40 case in iwl_mvm_send_sta_key.
This will document that the lack of a break is intentional.

Coverity: CID 1260023

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>