nrf: disable interrupts before running wfi

Also, run background tasks with interrupts disabled.  This ensures
that any buffers are drained, and if an interrupt arrives after
buffers are drained but before the WFI is executed then the WFI
instruction will return immediately.

This fixes adafruit#2855 on NRF.

Signed-off-by: Sean Cross <sean@xobs.io>

ports/nrf/supervisor/port.c

@@ -51,6 +51,7 @@
 #include "common-hal/rtc/RTC.h"
 #include "common-hal/neopixel_write/__init__.h"
 
+#include "shared-bindings/microcontroller/__init__.h"
 #include "shared-bindings/rtc/__init__.h"
 
 #ifdef CIRCUITPY_AUDIOBUSIO
@@ -247,7 +248,15 @@ void port_sleep_until_interrupt(void) {
         sd_app_evt_wait();
     } else {
         // Call wait for interrupt ourselves if the SD isn't enabled.
+        // Note that `wfi` should be called with interrupts disabled,
+        // to ensure that the queue is properly drained.  The `wfi`
+        // instruction will returned as long as an interrupt is
+        // available, even though the actual handler won't fire until
+        // we re-enable interrupts.
+        common_hal_mcu_disable_interrupts();
+        RUN_BACKGROUND_TASKS; // Drain any buffers remaining
         __WFI();
+        common_hal_mcu_enable_interrupts();
     }
 }