net: dsa: sja1105: Make two functions static

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: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>