dco_linux: fix case statement by using proper error value

dco_linux: fix case statement by using proper error value

A M_ERR constant accidentally slipped in as possible
netlink error value.

Substitute it with the actual code matching the following
error message.

Change-Id: I18df6ef659cab9525dd7847b7dd3950fc1895dd5
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250723062012.22781-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32269.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>