sync_modem: Fix race with fast or failed send().

If send completes before the first call to poll_send(), the driver could
get stuck in _sync_wait(). This had much less impact before rp2 port went
tickless, as _sync_wait(will_irq=True) calls machine.idle() which may not
wake very frequently on a tickless port.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <[email protected]>

micropython/lora/lora-sync/lora/sync_modem.py

@@ -42,8 +42,8 @@ def send(self, packet, tx_at_ms=None):
 
         tx = True
         while tx is True:
-            tx = self.poll_send()
             self._sync_wait(will_irq)
+            tx = self.poll_send()
         return tx
 
     def recv(self, timeout_ms=None, rx_length=0xFF, rx_packet=None):