hwmon: (w83781d) Fix W83782D support (NULL pointer dereference)

Commit 360782dd (hwmon: (w83781d) Stop
abusing struct i2c_client for ISA devices) broke W83782D support for
devices connected on the ISA bus. You will hit a NULL pointer
dereference as soon as you read any device attribute. Other devices,
and W83782D devices on the SMBus, aren't affected.

Reported-by: Michel Abraham
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Michel Abraham