hwmon: (it87) Properly handle wrong sensor type requests

Currently, if someone tries to set the thermal sensor type to an
unsupported value, subsequent accesses to the chip may temporarily
show the sensor in question as disabled. Use a temporary variable
and only update the cached value on success, to prevent such
confusion.

Signed-off-by: Jean Delvare <khali@linux-fr.org>

diff --cc drivers/hwmon/it87.c
Simple merge