efi: Use 16-bit unicode strings

At present we use wide characters for unicode but this is not necessary.
Change the code to use the 'u' literal instead. This helps to fix build
warnings for sandbox on rpi.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/include/efi_selftest.h b/include/efi_selftest.h
index 94ceb14..5340cef 100644
--- a/include/efi_selftest.h
+++ b/include/efi_selftest.h
@@ -16,7 +16,7 @@
 
 #define EFI_ST_SUCCESS 0
 #define EFI_ST_FAILURE 1
-#define EFI_ST_SUCCESS_STR L"SUCCESS"
+#define EFI_ST_SUCCESS_STR u"SUCCESS"
 
 /**
  * efi_st_printf() - print a message
@@ -111,7 +111,7 @@
  * efi_st_translate_code() - translate a scan code to a human readable string
  *
  * This function translates the scan code returned by the simple text input
- * protocol to a human readable string, e.g. 0x04 is translated to L"Left".
+ * protocol to a human readable string, e.g. 0x04 is translated to u"Left".
  *
  * @code:	scan code
  * Return:	Unicode string