ASoC: 88pm860x: array overflow in snd_soc_put_volsw_2r_st()

This is called from snd_ctl_elem_write() with user supplied data so we
need to add some bounds checking.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org