iwlwifi: mvm: remove sta key on wep ap

If WEP is used, no one removes the STA key upon STA
removal, leading to a situation where after 16
connect/disconnects - the AP could no longer decrypt
incoming data frames since iwl_mvm_set_fw_key_idx()
called from iwl_mvm_set_sta_key() during association
returns STA_KEY_IDX_INVALID, thus not setting the key
for that connecting STA.

Fix this by removing the key in the driver when the
STA is removed.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>