Fix compilation error

Author: datdenkikniet

src/dma/cache.rs

@@ -1,3 +1,4 @@
+#[cfg(feature = "ptp")]
 use crate::ptp::Timestamp;
 
 use super::PacketId;
@@ -52,6 +53,8 @@ impl Cache {
         }
     }
 
+    // NOTE(unused): this function is not used if not(feature = "ptp")
+    #[allow(unused)]
     pub fn id(&self) -> Option<PacketId> {
         if self.has_packet_id {
             Some(self.packet_id)

src/dma/rx/f_series_descriptor.rs

@@ -52,8 +52,6 @@ impl RxDescriptor {
             desc: RawDescriptor::new(),
             last: false,
             cache: Cache::new(),
-            #[cfg(not(feature = "ptp"))]
-            _padding: [0u8; 4],
         }
     }