net: hns3: modify mac statistics update process for compatibility

After querying mac statistics from firmware, driver copies data from
descriptors to struct mac_stats of hdev, and the number of copied data
is just according to the register number queried from firmware. There is
a problem that if the register number queried from firmware is larger
than data number of struct mac_stats, it will cause a copy overflow.

So if the firmware adds more mac statistics in later version, it is not
compatible with driver of old version.

To fix this problem, the number of copied data needs to be used the
minimum value between the register number queried from firmware and
data number of struct mac_stats.

The first descriptor has three data and there is one reserved, to
optimize the copy process, add this reserverd data to struct mac_stats.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>