hwmon: (w83791d) Convert macros to functions to avoid TOCTOU

In the Linux kernel, the following vulnerability has been resolved:

hwmon: (w83791d) Convert macros to functions to avoid TOCTOU

The macro FAN_FROM_REG evaluates its arguments multiple times. When used in lockless contexts involving shared driver data, this leads to Time-of-Check to Time-of-Use (TOCTOU) race conditions, potentially causing divide-by-zero errors.

Convert the macro to a static function. This guarantees that arguments are evaluated only once (pass-by-value), preventing the race conditions.

Additionally, in store_fan_div, move the calculation of the minimum limit inside the update lock. This ensures that the read-modify-write sequence operates on consistent data.

Adhere to the principle of minimal changes by only converting macros that evaluate arguments multiple times and are used in lockless contexts.