selftests: drv-net: csum: fix interface name for remote host

Use cfg.remote_ifname for arguments of remote command.
Without this UDP tests fail in NIPA where local interface
is called enp1s0 and remote enp0s4.

Fixes: 1d0dc857 ("selftests: drv-net: add checksum tests")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20250830183842.688935-1-kuba@kernel.org


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