iio: adc: cpcap-adc: remove redundant assignment to variable cal_data_diff

iio: adc: cpcap-adc: remove redundant assignment to variable cal_data_diff

The variable cal_data_diff is being assigned a value that is never read,
it is being re-assigned later on with a new value in both paths of an
if statement. The assignment is redundant, so remove it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220207135943.340629-1-colin.i.king@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>