iio: stmpe-adc: Reinit completion struct on begin conversion

iio: stmpe-adc: Reinit completion struct on begin conversion

In some cases, the wait_completion got interrupted. This caused the
error-handling to mutex_unlock the function. The before turned on
interrupt then got called anyway. In the ISR then completion() was
called causing wrong adc-values returned in a following adc-readout.

Reinitialise completion struct to make sure the counter is zero
when beginning a new adc-conversion.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>