scripts: generate_rust_analyzer: Handle sub-modules with no Makefile

More complex drivers might want to use modules to organize their Rust
code, but those module folders do not need a Makefile.
generate_rust_analyzer.py currently crashes on those. Fix it so that a
missing Makefile is silently ignored.

Link: https://github.com/Rust-for-Linux/linux/pull/883
Signed-off-by: Asahi Lina <lina@asahilina.net>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>