rust: phy: use to_result for error handling

Simplifies error handling by replacing the manual check
of the return value with the `to_result` helper.

Signed-off-by: Onur Özkan <[e-mail]>
Reviewed-by: Elle Rhumsaa <[e-mail]>
Reviewed-by: Trevor Gross <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

rust/kernel/net/phy.rs

···
         // SAFETY: `phydev` is pointing to a valid object by the type invariant of `Self`.
         // So it's just an FFI call.
         let ret = unsafe { bindings::phy_read_paged(phydev, page.into(), regnum.into()) };
-        if ret < 0 {
-            Err(Error::from_errno(ret))
-        } else {
-            Ok(ret as u16)
-        }
+
+        to_result(ret).map(|()| ret as u16)
     }
 
     /// Resolves the advertisements into PHY settings.