Revert "iio: accel: st_accel: remove redundant pointer pdata"

This reverts commit 585ed27d.

This removed code which was unused due to a bug in commit 7383d44b.
To fix this bug the code is needed. Thus this revert.

Signed-off-by: Michael Nosthoff <committed@heine.so>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>