iwlwifi: fix remove key error

Fix following error by sending synchronous command and waiting for the command
to complete.

mac80211-phy0: failed to remove key (0, ff:ff:ff:ff:ff:ff) from hardware (-16).

-16 is EBUSY error. The asynchronous command tests for STATUS_EXIT_PENDING
while interface is getting down and it returns -EBUSY error if set.
Changing the host command from asynchronous call to synchronous call
enables command to be run while interface is going down.
Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-sta.c

@@ -520,7 +520,7 @@ int iwl_send_static_wepkey_cmd(struct iwl_priv *priv, u8 send_if_empty)
 	struct iwl_host_cmd cmd = {
 		.id = REPLY_WEPKEY,
 		.data = wep_cmd,
-		.flags = CMD_ASYNC,
+		.flags = CMD_SYNC,
 	};
 
 	memset(wep_cmd, 0, cmd_size +