ASoC: 88pm860x: array overflow in snd_soc_put_volsw_2r_st()

commit d967967e8d1116fb38bad25e58714b5dddd03cca upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>