hwmon: (w83781d) Stop abusing struct i2c_client for ISA devices

Upcoming changes to the I2C part of the w83781d driver will cause ISA
devices to no longer have a struct i2c_client at hand. So, we must
stop (ab)using it now.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Wolfgang Grandegger <wg@grandegger.com>