net: hns3: fix some mac statistics is always 0 in device version V2

When driver queries the register number of mac statistics from firmware,
the old firmware runs in device version V2 only returns number of valid
registers, not include number of three reserved registers among of them.
It cause driver doesn't record the last three data when query mac
statistics.

To fix this problem, driver never query register number in device version
V2 and set it to a fixed value which include three reserved registers.

Fixes: c8af2887 ("net: hns3: add support pause/pfc durations for mac statistics")
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>