ethtool: Expand ethtool_cmd.speed to 32 bits

Introduce the speed_hi field to ethtool_cmd, using the reserved space,
to expand the speed field to 2^32 Megabits/second.

Making this field expansion now gives us plenty of time to fix up the
user-space pieces that use SIOCETHTOOL before hardware faster than 64
Gb/s is available.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>