hwmon: (g762) Fix overflows and crash seen when writing limit attributes

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>

drivers/hwmon/g762.c

@@ -193,14 +193,17 @@ static inline unsigned int rpm_from_cnt(u8 cnt, u32 clk_freq, u16 p,
  * Convert fan RPM value from sysfs into count value for fan controller
  * register (FAN_SET_CNT).
  */
-static inline unsigned char cnt_from_rpm(u32 rpm, u32 clk_freq, u16 p,
+static inline unsigned char cnt_from_rpm(unsigned long rpm, u32 clk_freq, u16 p,
 					 u8 clk_div, u8 gear_mult)
 {
-	if (!rpm)         /* to stop the fan, set cnt to 255 */
+	unsigned long f1 = clk_freq * 30 * gear_mult;
+	unsigned long f2 = p * clk_div;
+
+	if (!rpm)	/* to stop the fan, set cnt to 255 */
 		return 0xff;
 
-	return clamp_val(((clk_freq * 30 * gear_mult) / (rpm * p * clk_div)),
-			 0, 255);
+	rpm = clamp_val(rpm, f1 / (255 * f2), ULONG_MAX / f2);
+	return DIV_ROUND_CLOSEST(f1, rpm * f2);
 }
 
 /* helper to grab and cache data, at most one time per second */