iio: stmpe-adc: Use wait_for_completion_timeout

Use wait_for_completion_timeout instead of
wait_for_completion_interuptible_timeout.

The interruptible variant gets constantly interrupted if a user
program is compiled with the -pg option.
The killable variant was not used due to the fact that a second
program, reading on this device, that gets killed is then also killing
that wait.

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