cgxb4: Stop using ethtool SPEED_* constants

ethtool speed values are just numbers of megabits and there is no need
to add SPEED_40000.  To be consistent, use integer constants directly
for all speeds.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>