linux/generic.nix: Fix CONFIG_RUST

I thought it was module based, but it wasn't.

Fixes https://github.com/NixOS/nixpkgs/pull/306790#issuecomment-2092485576

> This has broken [...] Rust support

pkgs/os-specific/linux/kernel/generic.nix

···
     config = {
       CONFIG_MODULES = "y";
       CONFIG_FW_LOADER = "m";
-      CONFIG_RUST = lib.mkIf withRust "y";
+      CONFIG_RUST = if withRust then "y" else "n";
     };
   });