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_loader/efi_boottime.c b/lib/efi_loader/efi_boottime.c
index 37b9c68..1ea04de 100644
--- a/lib/efi_loader/efi_boottime.c
+++ b/lib/efi_loader/efi_boottime.c
@@ -3778,7 +3778,7 @@
 	.create_event_ex = efi_create_event_ex,
 };
 
-static u16 __efi_runtime_data firmware_vendor[] = L"Das U-Boot";
+static u16 __efi_runtime_data firmware_vendor[] = u"Das U-Boot";
 
 struct efi_system_table __efi_runtime_data systab = {
 	.hdr = {