Enable Werror on some network drivers

Change-Id: Iff3a167da4c7a639f229556c501c2b71133fd8dc
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5308
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

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.