Fix uart race condition dropping bytes on NRF52

In nordic_nrf5_uart_event_handler if the events NRF_UARTE_EVENT_ENDRX
and NRF_UARTE_EVENT_RXSTARTED become pending after the check for
NRF_UARTE_EVENT_ENDRX but before the check for
NRF_UARTE_EVENT_RXSTARTED the RX DMA buffers will be setup incorrectly
by nordic_nrf5_uart_event_handler_rxstarted because active_bank hasn't
been updated. This cause dropped and incorrect data.

This patch fixes that problem by adding a second check for
NRF_UARTE_EVENT_ENDRX after checking for NRF_UARTE_EVENT_RXSTARTED
and skipping processing if NRF_UARTE_EVENT_ENDRX is set. The
subsequent interrupt will process both in the correct order. This
ensures that these events cannot be handled out of order and thus fixes
the corruption.

Author: c1728p9

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/serial_api.c

@@ -727,8 +727,14 @@ static void nordic_nrf5_uart_event_handler(int instance)
         }
     }
 
-    /* Rx DMA buffer has been armed. */
-    if (nrf_uarte_event_check(nordic_nrf5_uart_register[instance], NRF_UARTE_EVENT_RXSTARTED))
+    /* Rx DMA buffer has been armed.
+     *
+     * Warning - Do not process NRF_UARTE_EVENT_RXSTARTED if NRF_UARTE_EVENT_ENDRX is pending.
+     * NRF_UARTE_EVENT_RXSTARTED must be processed first or nordic_nrf5_uart_event_handler_rxstarted
+     * will setup the wrong DMA buffer and cause data to be lost.
+     */
+    if (nrf_uarte_event_check(nordic_nrf5_uart_register[instance], NRF_UARTE_EVENT_RXSTARTED) &&
+            !nrf_uarte_event_check(nordic_nrf5_uart_register[instance], NRF_UARTE_EVENT_ENDRX))
     {
         nrf_uarte_event_clear(nordic_nrf5_uart_register[instance], NRF_UARTE_EVENT_RXSTARTED);