hwmon: Fix debug messages in w83781d

Fix debug messages in w83781d at detection time. We can't use dev_dbg()
on an i2c client's device before calling i2c_attach_client() on that
client.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>