net/fungible: Avoid -Wflex-array-member-not-at-end warning

Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warning:
drivers/net/ethernet/fungible/funcore/fun_dev.c:550:43: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/ZrDwEugW7DR/FlP5@cuteSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/fungible/funcore/fun_dev.c

@@ -546,17 +546,14 @@ int fun_bind(struct fun_dev *fdev, enum fun_admin_bind_type type0,
 	     unsigned int id0, enum fun_admin_bind_type type1,
 	     unsigned int id1)
 {
-	struct {
-		struct fun_admin_bind_req req;
-		struct fun_admin_bind_entry entry[2];
-	} cmd = {
-		.req.common = FUN_ADMIN_REQ_COMMON_INIT2(FUN_ADMIN_OP_BIND,
-							 sizeof(cmd)),
-		.entry[0] = FUN_ADMIN_BIND_ENTRY_INIT(type0, id0),
-		.entry[1] = FUN_ADMIN_BIND_ENTRY_INIT(type1, id1),
-	};
+	DEFINE_RAW_FLEX(struct fun_admin_bind_req, cmd, entry, 2);
+
+	cmd->common = FUN_ADMIN_REQ_COMMON_INIT2(FUN_ADMIN_OP_BIND,
+						 __struct_size(cmd));
+	cmd->entry[0] = FUN_ADMIN_BIND_ENTRY_INIT(type0, id0);
+	cmd->entry[1] = FUN_ADMIN_BIND_ENTRY_INIT(type1, id1);
 
-	return fun_submit_admin_sync_cmd(fdev, &cmd.req.common, NULL, 0, 0);
+	return fun_submit_admin_sync_cmd(fdev, &cmd->common, NULL, 0, 0);
 }
 EXPORT_SYMBOL_GPL(fun_bind);