net: hns3: add header file hns3_ethtoo.h

Add a new file hns3_ethtool.h, and move struct type definitions from
hns3_ethtool.c to hns3_ethtool.h.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>