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/lib/efi_selftest/efi_selftest_devicepath.c b/lib/efi_selftest/efi_selftest_devicepath.c
index d87b9f7..8a22a5d 100644
--- a/lib/efi_selftest/efi_selftest_devicepath.c
+++ b/lib/efi_selftest/efi_selftest_devicepath.c
@@ -319,7 +319,7 @@
 			{ DEVICE_PATH_TYPE_MEDIA_DEVICE,
 			  DEVICE_PATH_SUB_TYPE_FILE_PATH,
 			  sizeof(struct efi_device_path) + 12},
-			L"u-boot.bin",
+			u"u-boot.bin",
 		};
 	u16 *string;
 	efi_status_t ret;