staging: iio: adc: ad7606: Use wait-for-completion handler

This patch replaces the use of wait_event_interruptible() with
wait_for_completion_timeout() when reading the result of a single
conversion. In this way, if the interrupt never occurs, the program will
not remain blocked.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>