net: dsa: ocelot: fix "should it be static?" warnings

Fix following sparse warnings:
drivers/net/dsa/ocelot/felix.c:351:6: warning: symbol 'felix_txtstamp' was not declared. Should it be static?

Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>