hwmon: (ibmpex) fix use-after-free in high/low store

hwmon: (ibmpex) fix use-after-free in high/low store

The ibmpex_high_low_store() function retrieves driver data using
dev_get_drvdata() and uses it without validation. This creates a race
condition where the sysfs callback can be invoked after the data
structure is freed, leading to use-after-free.

Fix by adding a NULL check after dev_get_drvdata(), and reordering
operations in the deletion path to prevent TOCTOU.

Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Reported-by: Junrui Luo <moonafterrain@outlook.com>
Fixes: 57c7c3a0fdea ("hwmon: IBM power meter driver")
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Link: https://lore.kernel.org/r/MEYPR01MB7886BE2F51BFE41875B74B60AFA0A@MEYPR01MB7886.ausprd01.prod.outlook.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>