net: hns3: refine function hclge_cfg_mac_speed_dup_hw()

To reuse the code of converting speed of driver to speed of firmware in
function hclge_cfg_mac_speed_dup_hw(), encapsulate them into a new
function hclge_convert_to_fw_speed().

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