net: hns3: add new function hclge_get_speed_bit()

Currently, function hclge_check_port_speed() uses switch/case statement
to get speed bit according to speed. To reuse this part of code and
improve code readability and maintainability, add a new function
hclge_get_speed_bit() to get speed bit according to map relationship
of speed and speed bit defined in array speed_bit_map.

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