ethernet: hisilicon: hns: hns_dsaf_misc: fix a possible array overflow in hns_dsaf_ge_srst_by_port()

The if statement:
  if (port >= DSAF_GE_NUM)
        return;

limits the value of port less than DSAF_GE_NUM (i.e., 8).
However, if the value of port is 6 or 7, an array overflow could occur:
  port_rst_off = dsaf_dev->mac_cb[port]->port_rst_off;

because the length of dsaf_dev->mac_cb is DSAF_MAX_PORT_NUM (i.e., 6).

To fix this possible array overflow, we first check port and if it is
greater than or equal to DSAF_MAX_PORT_NUM, the function returns.
Reported-by: NTOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: NTeng Qi <starmiku1207184332@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>