netdev: Get rid of netdev_dev.

The distinction between struct netdev_dev and struct netdev has always
been confusing.  Now that previous commits have eliminated all interesting
state from struct netdev, this commit deletes it and renames struct
netdev_dev to take its place.  Now the situation makes much more sense and
I won't have to continue making embarrassed explanations in the future.

Good riddance.

Signed-off-by: Ben Pfaff <[email protected]>