bswap: Fix accessors syntax in comment

All accessors that have an endian infix DO have an underscore between
{size} and {endian}.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <155119086741.1037569.12734854713022304642.stgit@bahia.lan>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>