iio: dac: ti-dac5571 remove redundant variable 'shift'

Variable shift is being assigned but is never used hence it is
redundant and can be removed.

Cleans up two clang warnings:
warning: variable ‘shift’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>