Revert "nforce: Disable MTU changes."

This reverts commit 25742b78fb8de17c76b0c8dfefa34441c9a88e95.

It was confirmed the MTU change introduction was unrelated to the
problems seen in this ticket.

nforce: Disable MTU changes.

Strange KDLs showed up on at least one system with this driver after
the enabling of jumbo frames, so it seems probable that was the cause.
Disable MTU changes to hopefully prevent this problem for now.

Should hopefully stop #17766.

drivers/network: Move ethernet drivers to an "ether" subdirectory.

Now that there is also a "tap" driver in here, and the WiFi drivers
are also in a subdirectory, moving the physical ethernet device drivers
to a subdirectory also makes sense.

(Briefly) discussed on the mailing lists.