staging:iio:cdc:ad7150: Timeout register covers both directions so both need updating

The timeout is treated as one single value, but the datasheet describes
it as two 4 bit values, one for each direction of event.
As such change the driver to support the separate directions.
Also add limit checking to ensure it fits within the 4 bits.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-5-jic23@kernel.org