The result of an integer divide by an unsigned is undefined.
This causes unexpected results when writing negative values
into the limit registers.

Maintain the shunt_resistors variables as signed integer to avoid
the problem. Also, for simplicity and ease of use, clamp shunt
resistor value on writes instead of rejecting bad values.

Cc: Andrew F. Davis <afd@ti.com>
Acked-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>