net: lan743x_ptp: make function lan743x_ptp_set_sync_ts_insert() static

Fixes the following sparse warning:

drivers/net/ethernet/microchip/lan743x_ptp.c:980:6: warning:
 symbol 'lan743x_ptp_set_sync_ts_insert' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>