Fix SPI initialization for NRF52 series

New changes to Mbed error reporting in 5.9 exposed bug in SPI
driver where an instance was uninitialized twice which triggered
an ASSERT.

This fix keeps track of which instance has been initialized and
only calls uninit when it is safe.

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/spi_api.c

@@ -52,6 +52,9 @@ static const nrf_drv_spi_t nordic_nrf5_spi_instance[3] = {
     NRF_DRV_SPI_INSTANCE(2)
 };
 
+/* Keep track of which instance has been initialized. */
+static bool nordic_nrf5_spi_initialized[3] = { false, false, false };
+
 /* Forware declare interrupt handler. */
 #if DEVICE_SPI_ASYNCH
 static void nordic_nrf5_spi_event_handler(nrf_drv_spi_evt_t const *p_event, void *p_context);
@@ -93,8 +96,10 @@ static void spi_configure_driver_instance(spi_t *obj)
         /* Update applied, reset flag. */
         spi_inst->update = false;
 
-        /* clean up and initialize peripheral. */
-        nrf_drv_spi_uninit(&nordic_nrf5_spi_instance[instance]);
+        /* Clean up and uninitialize peripheral if already initialized. */
+        if (nordic_nrf5_spi_initialized[instance]) {
+            nrf_drv_spi_uninit(&nordic_nrf5_spi_instance[instance]);
+        }
 
 #if DEVICE_SPI_ASYNCH
         /* Set callback handler in asynchronous mode. */
@@ -107,6 +112,10 @@ static void spi_configure_driver_instance(spi_t *obj)
         /* Set callback handler to NULL in synchronous mode. */
         nrf_drv_spi_init(&nordic_nrf5_spi_instance[instance], &(spi_inst->config), NULL, NULL);
 #endif
+
+        /* Mark instance as initialized. */
+        nordic_nrf5_spi_initialized[instance] = true;
+
         /* Claim ownership of peripheral. */
         object_owner_spi2c_set(instance, obj);
     }
@@ -201,6 +210,9 @@ void spi_free(spi_t *obj)
 
     /* Use driver uninit to free instance. */
     nrf_drv_spi_uninit(&nordic_nrf5_spi_instance[instance]);
+
+    /* Mark instance as uninitialized. */
+    nordic_nrf5_spi_initialized[instance] = false;
 }
 
 /** Configure the SPI format