iio: accel: st_accel: remove redundant pointer pdata

Pointer pdata is being assigned but it is never being used, hence
it is redundant and can be removed. Cleans up clang warning:

drivers/iio/accel/st_accel_core.c:952:3: warning: Value stored to 'pdata'
is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>