Improve pin stub so servo code doesn't explode.

See #76

Author: microbit-matt-hillsdon

src/microbithal_js.c

@@ -113,25 +113,9 @@ int microbit_hal_pin_set_analog_period_us(int pin, int period) {
         mp_js_hal_audio_period_us(period);
         return 0;
     }
-
-    /*
-    // Calling setAnalogPeriodUs requires the pin to be in analog-out mode.  So
-    // test for this mode by first calling getAnalogPeriodUs, and if it fails then
-    // attempt to configure the pin in analog-out mode by calling setAnalogValue.
-    if ((ErrorCode)pin_obj[pin]->getAnalogPeriodUs() == DEVICE_NOT_SUPPORTED) {
-        if (pin_obj[pin]->setAnalogValue(0) != DEVICE_OK) {
-            return -1;
-        }
-    }
-
-    // Set the analog period.
-    if (pin_obj[pin]->setAnalogPeriodUs(period) == DEVICE_OK) {
-        return 0;
-    } else {
-        return -1;
-    }
-    */
-    return -1;
+    // We don't support other pins yet but more practical to claim success as
+    // this allows programs using servos to otherwise function.
+    return 0;
 }
 
 int microbit_hal_pin_get_analog_period_us(int pin) {