staging:iio:dac:ad5504: Use strtobool for boolean values

Use strtobool for parsing the powerdown value instead of strict_strtol, since
the powerdown attribute is a boolean value.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>