hwmon: (aspeed-g6-pwm-tach) Guard fan RPM calculation against divide-by-zero

hwmon: (aspeed-g6-pwm-tach) Guard fan RPM calculation against divide-by-zero

Sashiko reports:

In the aspeed-g6-pwm-tacho driver, the aspeed_tach_val_to_rpm() function
calculates the fan RPM using the tachometer value. However, it does not
check if the tachometer value is zero before performing the division.

If the hardware reports a tachometer value of 0 (which can happen due to
an extremely fast pulse, a stuck edge, or a hardware glitch), the
calculated tach_div evaluates to 0. The subsequent call to do_div() with
tach_div as the divisor triggers a divide-by-zero exception, leading to
a kernel panic.

Check the divisor against zero to fix the problem.

Fixes: 7e1449cd15d1 ("hwmon: (aspeed-g6-pwm-tacho): Support for ASPEED g6 PWM/Fan tach")
Cc: Billy Tsai <billy_tsai@aspeedtech.com>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>