i8k: Make fan module parameters an unsigned

Setting negative fan multiplier or maximal fan speed does make any sense and
can cause problems. So ensure that negative values will not be accepted.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>