riscv: fix the wrong swap value register

Not s2 register, t1 register is correct
Fortunately, it works because t1 register has a garbage value

Signed-off-by: Brad Kim <brad.kim@semifive.com>
Reviewed-by: Lukas Auer <lukas@auer.io>
Reviewed-by: Leo Liang <ycliang@andestech.com>