netlink: specs: mptcp: fix if-idx attribute type

This attribute is used as a signed number in the code in pm_netlink.c:

  nla_put_s32(skb, MPTCP_ATTR_IF_IDX, ssk->sk_bound_dev_if))

The specs should then reflect that. Note that other 'if-idx' attributes
from the same .yaml file use a signed number as well.

Fixes: bc8aeb2 ("Documentation: netlink: add a YAML spec for mptcp")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250908-net-mptcp-misc-fixes-6-17-rc5-v1-1-5f2168a66079@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

Documentation/netlink/specs/mptcp_pm.yaml

@@ -256,7 +256,7 @@ attribute-sets:
         type: u32
       -
         name: if-idx
-        type: u32
+        type: s32
       -
         name: reset-reason
         type: u32