iio: dac: ad5758: remove set but not used variable 'dc_dc_mode'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/iio/dac/ad5758.c: In function ad5758_write_powerdown:
drivers/iio/dac/ad5758.c:585:15: warning: variable dc_dc_mode set but not used [-Wunused-but-set-variable]

It is not used any more since
commit edde9452 ("iio: dac: ad5758: Modifications for new revision")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>