Properly clear/initialize TX descriptors.

Unclear why, but this value does not seem to be zeroed correctly on startup, which
can cause TX to start without the descriptors being written correctly.

Author: datdenkikniet

src/dma/desc.rs

@@ -35,6 +35,10 @@ impl Descriptor {
         }
     }
 
+    pub unsafe fn clear(&mut self) {
+        (0..DESC_SIZE).for_each(|i| self.write(i, 0));
+    }
+
     fn r(&self, n: usize) -> &RO<u32> {
         let ro = &self.desc.deref()[n] as *const _ as *const RO<u32>;
         unsafe { &*ro }

src/dma/tx/descriptor.rs

@@ -171,6 +171,9 @@ impl RingDescriptor for TxDescriptor {
 
         self.buffer1 = buffer as u32;
         self.next_descriptor = next_desc_addr;
+        unsafe {
+            self.desc.clear();
+        }
     }
 }