Fix nRF UART reset

disable only turns off ENABLE but doesn't set the init tracking that
nrfx uses. uninit hangs if ENABLE is off and is called because it
waits forever for TX to stop.

Author: tannewt

ports/nrf/common-hal/busio/UART.c

@@ -124,7 +124,7 @@ static void uart_callback_irq (const nrfx_uarte_event_t * event, void * context)
 
 void uart_reset(void) {
     for (size_t i = 0 ; i < MP_ARRAY_SIZE(nrfx_uartes); i++) {
-        nrf_uarte_disable(nrfx_uartes[i].p_reg);
+        nrfx_uarte_uninit(&nrfx_uartes[i]);
     }
 }
 
@@ -171,7 +171,6 @@ void common_hal_busio_uart_construct (busio_uart_obj_t *self,
         }
     };
 
-    nrfx_uarte_uninit(self->uarte);
     _VERIFY_ERR(nrfx_uarte_init(self->uarte, &config, uart_callback_irq));
 
     // Init buffer for rx