rust: auxiliary: consider auxiliary devices always have a parent

An auxiliary device is guaranteed to always have a parent device (both
in C and Rust), hence don't return an Option<&auxiliary::Device> in
auxiliary::Device::parent().

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

samples/rust/rust_driver_auxiliary.rs

@@ -68,7 +68,7 @@ impl pci::Driver for ParentDriver {
 
 impl ParentDriver {
     fn connect(adev: &auxiliary::Device) -> Result<()> {
-        let parent = adev.parent().ok_or(EINVAL)?;
+        let parent = adev.parent();
         let pdev: &pci::Device = parent.try_into()?;
 
         let vendor = pdev.vendor_id();