selftests: ncdevmem: Remove default arguments

To make it clear what's required and what's not. Also, some of the
values don't seem like a good defaults; for example eth1.

Move the invocation comment to the top, add missing -s to the client
and cleanup the client invocation a bit to make more readable.

Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241107181211.3934153-6-sdf@fomichev.me


Signed-off-by: Jakub Kicinski <kuba@kernel.org>