efi_loader: use u16* for file name

UTF-16 strings in our code should all be u16 *. Fix an inconsistency for
file names which may lead to a warning for printf("%ls", ).

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>