Fix sparse warnings:

drivers/net/dsa/sja1105/sja1105_main.c:1848:6:
 warning: symbol 'sja1105_port_rxtstamp' was not declared. Should it be static?
drivers/net/dsa/sja1105/sja1105_main.c:1869:6:
 warning: symbol 'sja1105_port_txtstamp' was not declared. Should it be static?
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Tested-by: NVladimir Oltean <olteanv@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>