ethtool: cosmetics: enforce const-ness in ethtool_cmd_speed

The 'ep' argument of ethtool_cmd_speed is not altered: advertise it in
protoype. +Indentation fix. Also add comments to advise using the
ethtool_cmd_speed API to get/set the link speed.

Signed-off-by: David Decotigny <decot@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>