nrf: avoid too high priority interrupt on SoftDevice

The highest priority interrupts are reserved by the SoftDevice, the
SoftDevice will refuse to start if any of them are used for other
purposes.

src/machine/machine_nrf.go

@@ -145,7 +145,7 @@ func (p Pin) SetInterrupt(change PinChange, callback func(Pin)) error {
 
 	// Set and enable the GPIOTE interrupt. It's not a problem if this happens
 	// more than once.
-	interrupt.New(nrf.IRQ_GPIOTE, func(interrupt.Interrupt) {
+	intr := interrupt.New(nrf.IRQ_GPIOTE, func(interrupt.Interrupt) {
 		for i := range nrf.GPIOTE.EVENTS_IN {
 			if nrf.GPIOTE.EVENTS_IN[i].Get() != 0 {
 				nrf.GPIOTE.EVENTS_IN[i].Set(0)
@@ -153,7 +153,9 @@ func (p Pin) SetInterrupt(change PinChange, callback func(Pin)) error {
 				pinCallbacks[i](pin)
 			}
 		}
-	}).Enable()
+	})
+	intr.SetPriority(0x40) // interrupt priority 2 (highest priority not reserved by the SoftDevice)
+	intr.Enable()
 
 	// Everything was configured correctly.
 	return nil