libboard_zynq: remove unneeded Uart flush

libboard_zynq/src/uart/mod.rs

@@ -192,15 +192,14 @@ impl Uart {
         self.regs.channel_sts.read().txfull()
     }
 
-    pub fn tx_fifo_empty(&self) -> bool {
-        self.regs.channel_sts.read().txempty()
+    pub fn tx_idle(&self) -> bool {
+        let status = self.regs.channel_sts.read();
+        status.txempty() && !status.tactive()
     }
 }
 
 impl fmt::Write for Uart {
     fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
-        while !self.tx_fifo_empty() {}
-
         for b in s.bytes() {
             self.write_byte(b);
         }