Merge pull request #5913 from DBS06/efm32_flash_api

Add missing EFM32 HAL flash init/deinit function calls

targets/TARGET_Silicon_Labs/TARGET_EFM32/flash_api.c

@@ -35,6 +35,7 @@
 int32_t flash_init(flash_t *obj)
 {
     (void)obj;
+    MSC_Init();
     return 0;
 }
 
@@ -46,6 +47,7 @@ int32_t flash_init(flash_t *obj)
 int32_t flash_free(flash_t *obj)
 {
     (void)obj;
+    MSC_Deinit();
     return 0;
 }