net/ncsi: Make local function ncsi_get_filter() static

Fixes the following sparse warnings:

net/ncsi/ncsi-manage.c:41:5: warning:
 symbol 'ncsi_get_filter' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>