staging:iio:ad2s1210: Remove old_data from ad2s1210_state

The variable old_data is a bool type, which only receives the value
'true' in the function ad2s1210_config_write and ad2s1210_config_read.
There is no other use for this variable. This patch removes old_data
from the ad2s1210_state and from all the function that use it.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>