iio: hid-sensor-prox: Restore lost scale assignments

The variables `scale_pre_decml`, `scale_post_decml`, and `scale_precision`
were assigned in commit d68c592 ("iio: hid-sensor-prox: Fix scale not
correct issue"), but due to a merge conflict in
commit 9c15db9 ("Merge tag 'iio-for-5.13a' of
https://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio into staging-next"),
these assignments were lost.

Add back lost assignments and replace `st->prox_attr` with
`st->prox_attr[0]` because commit 596ef5c ("iio: hid-sensor-prox: Add
support for more channels") changed `prox_attr` to an array.

Cc: stable@vger.kernel.org # 5.13+
Fixes: 9c15db9 ("Merge tag 'iio-for-5.13a' of https://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio into staging-next")
Signed-off-by: Zhang Lixu <lixu.zhang@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://patch.msgid.link/20250331055022.1149736-2-lixu.zhang@intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: lixuzha

drivers/iio/light/hid-sensor-prox.c

@@ -257,6 +257,11 @@ static int prox_parse_report(struct platform_device *pdev,
 
 	st->num_channels = index;
 
+	st->scale_precision = hid_sensor_format_scale(hsdev->usage,
+						      &st->prox_attr[0],
+						      &st->scale_pre_decml,
+						      &st->scale_post_decml);
+
 	return 0;
 }