mfd: wm831x-auxadc: Prevent use after free in wm831x_auxadc_read_irq()

The "req" struct is always added to the "wm831x->auxadc_pending" list,
but it's only removed from the list on the success path.  If a failure
occurs then the "req" struct is freed but it's still on the list,
leading to a use after free.

Fixes: 78bb3688 ("mfd: Support multiple active WM831x AUXADC conversions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>