hwmon: (emc2103) Add locking to avoid TOCTOU

hwmon: (emc2103) Add locking to avoid TOCTOU

The functions fan1_input_show and fan1_target_show check shared data for
zero before using it as a divisor. These accesses are currently
lockless. If the data changes to zero between the check and the
division, it causes a divide-by-zero error.

Explicitly acquire the update lock around these checks and calculations
to ensure the data remains stable, preventing Time-of-Check to
Time-of-Use (TOCTOU) race conditions.

Link: https://lore.kernel.org/all/CALbr=LYJ_ehtp53HXEVkSpYoub+XYSTU8Rg=o1xxMJ8=5z8B-g@mail.gmail.com/
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Link: https://lore.kernel.org/r/20251124165508.4667-1-hanguidong02@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>