net: bridge: assign path_cost for 2.5G and 5G link speed

As 2.5G, 5G ethernet ports are more common and affordable,
these ports are being used in LAN bridge devices.
STP port_cost() is missing path_cost assignment for these link speeds,
causes highest cost 100 being used.
This result in lower speed port being picked
when there is loop between 5G and 1G ports.

Original path_cost: 10G=2, 1G=4, 100m=19, 10m=100
Adjusted path_cost: 10G=2, 5G=3, 2.5G=4, 1G=5, 100m=19, 10m=100
                    speed greater than 10G = 1

Signed-off-by: Steven Hsieh <steven.hsieh@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>