HSI: ssi_protocol: Fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/hsi/clients/ssi_protocol.c:904:6: warning:
 symbol 'ssip_port_event' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

drivers/hsi/clients/ssi_protocol.c

@@ -901,7 +901,7 @@ static void ssip_tx_data_complete(struct hsi_msg *msg)
 	ssip_free_data(msg);
 }
 
-void ssip_port_event(struct hsi_client *cl, unsigned long event)
+static void ssip_port_event(struct hsi_client *cl, unsigned long event)
 {
 	switch (event) {
 	case HSI_EVENT_START_RX: