netdev-linux: Fix build failure with old kernel headers.

netdev-linux: Fix build failure with old kernel headers.

The "speed_hi" member was only introduced in 2.6.27, so builds against
older kernel headers failed.

speed_hi is fully backward compatible with older kernels, because older
kernels always set it to 0, so we could easily introduce a compatibility
layer here, but in fact I don't know of any OVS users who have interfaces
faster than 65.5 Gb/s, so it's hardly urgent.

Signed-off-by: Ben Pfaff <blp@nicira.com>