At present we are incorrectly setting the register to 0x1 to power up
the ADC. Since it is an active high power down register, we need to set
the register to 0x0 to actually power up. Conversely, writing 0x1 to the
register powers it down.

This commit adds a couple of helpers to make the code clearer and then
use them to do the power-up/power-down properly.

Fixes: 1664f6a5 ("iio: adc: Cosmic Circuits 10001 ADC driver")
Signed-off-by: Naidu Tellapati <naidu.tellapati@imgtec.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>