iio: dac: ad5755: Remove redundant code

The if check is redundant as the value obtained from
iio_device_register() is already in the required format.
Error messages are already printed by iio_device_register();
hence not needed.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>