Don't check for txstopped when writing to UART.

It seems like endtx and txstopped race, and if we see txstopped first
we return an error, causing writeln!(...).unwrap() to panic.

This changes makes the UART work reliably on the nRF9160-PDK.

Author: Jonathan Pallant (42 Technology)

nrf-hal-common/src/uarte.rs

@@ -135,31 +135,26 @@ where
             unsafe { w.bits(1) });
 
         // Wait for transmission to end.
-        let mut endtx;
-        let mut txstopped;
-        loop {
-            endtx = self.0.events_endtx.read().bits() != 0;
-            txstopped = self.0.events_txstopped.read().bits() != 0;
-            if endtx || txstopped {
-                break;
-            }
+        while self.0.events_endtx.read().bits() == 0 {
+            // TODO: Do something here which uses less power. Like `wfi`.
         }
 
         // Conservative compiler fence to prevent optimizations that do not
         // take in to account actions by DMA. The fence has been placed here,
         // after all possible DMA actions have completed.
         compiler_fence(SeqCst);
 
-        if txstopped {
-            return Err(Error::Transmit);
-        }
-
         // Lower power consumption by disabling the transmitter once we're
         // finished.
         self.0.tasks_stoptx.write(|w|
             // `1` is a valid value to write to task registers.
             unsafe { w.bits(1) });
 
+        // Wait for transmitter to stop.
+        while self.0.events_txstopped.read().bits() == 0 {
+            // Spin
+        }
+
         Ok(())
     }