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>