iio: stmpe-adc: Remove unnecessary assignment

Remove unnecessary assignment. This could potentially cause an issue, if
the wait function runs into a timeout. Furthermore is this assignment also
not there in stmpe_read_temp()

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