netdevsim: fix duplicated debugfs directory

The "ethtool" debugfs directory holds per-netdev knobs, so move
it from the device instance directory to the port directory.

This fixes the following warning when creating multiple ports:

 debugfs: Directory 'ethtool' with parent 'netdevsim1' already present!

Fixes: ff1f7c17

 ("netdevsim: add pause frame stats")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>