iio: ad5504: Fix setting power-down state

The power-down mask of the ad5504 is actually a power-up mask. Meaning if
a bit is set the corresponding channel is powered up and if it is not set
the channel is powered down.

The driver currently has this the wrong way around, resulting in the
channel being powered up when requested to be powered down and vice versa.

Fixes: 3bbbf150

 ("staging:iio:dac:ad5504: Use strtobool for boolean values")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201209104649.5794-1-lars@metafoo.de
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>