hwmon: (max6697) fix MAX6581 ideality

Without this patch, the values for ideality (register 0x4b) and ideality
selection mask (register 0x4c) are inverted.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Cc: stable@vger.kernel.org # 3.9+
Signed-off-by: Guenter Roeck <linux@roeck-us.net>