hinic: make symbol 'dump_mox_reg' static

Fix sparse warnings:

drivers/net/ethernet/huawei/hinic/hinic_hw_mbox.c:601:6: warning: symbol 'dump_mox_reg' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>