iio: adc: stmpe-adc: Fix wait_for_completion_timeout return value check

iio: adc: stmpe-adc: Fix wait_for_completion_timeout return value check

wait_for_completion_timeout() returns unsigned long not long.
it returns 0 if timed out, and positive if completed.
The check for <= 0 is ambiguous and should be == 0 here
indicating timeout which is the only error case

Fixes: e813dde6f833 ("iio: stmpe-adc: Use wait_for_completion_timeout")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Philippe Schenker <philippe.schenker@toradex.com>
Link: https://lore.kernel.org/r/20220412065150.14486-1-linmq006@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>