Fix overflows seen when writing into fan speed limit attributes.
Also fix crash due to division by zero, seen when certain very
large values (such as 2147483648, or 0x80000000) are written
into fan speed limit attributes.

Fixes: 594fbe71 ("Add support for GMT G762/G763 PWM fan controllers")
Cc: Arnaud Ebalard <arno@natisbad.org>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>