Fix UART initialization for NRF52

Delayed initialization can cause problems when both UARTE instances
are in use. This change causes each UART object to initialize the
underlying UARTE instance immediately.

Author: Marcus Chang

targets/TARGET_NORDIC/TARGET_NRF5x/serial_api.c

@@ -1046,24 +1046,22 @@ void serial_init(serial_t *obj, PinName tx, PinName rx)
         uart_object->rx = rx;
     }
 
-    /* Set default parity and baud rate. */
+    /* Set default parity, baud rate, and callback handler. */
     uart_object->parity = NRF_UART_PARITY_EXCLUDED;
     uart_object->baudrate = NRF_UART_BAUDRATE_9600;
     uart_object->cts = NRF_UART_PSEL_DISCONNECTED;
     uart_object->rts = NRF_UART_PSEL_DISCONNECTED;
     uart_object->hwfc = NRF_UART_HWFC_DISABLED;
+    uart_object->handler = 0;
 
     /* The STDIO object is stored in this file. Set the flag once initialized. */
     if (obj == &stdio_uart) {
         stdio_uart_inited = 1;
     }
 
-    /* Initializing the serial object does not make it the owner of an instance. 
-     * Only when the serial object is being used will the object take ownership 
-     * over the instance.
-     */
+    /* Take ownership and configure UART. */
     uart_object->update = true;
-    uart_object->handler = 0;
+    nordic_nrf5_serial_configure(obj);
 }
 
 /** Release the serial peripheral, not currently invoked. It requires further