net: atm: remove support for ZeitNet ZN122x ATM devices

This driver received nothing but automated fixes in the last 15 years.
Since it's using virt_to_bus it's unlikely to be used on any modern
platform.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>