net: hns3: device specifications add number of mac statistics

Currently, driver queries number of mac statistics before querying mac
statistics. As the number of mac statistics is a fixed value in firmware,
it is redundant to query this number everytime before querying mac
statistics, it can just be queried once in initialization process and
saved in device specifications.

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