i2c/vt8231: Remove superfluous initialization

Remove a superfluous initialization from the vt8231 hwmon driver; the
i2c core does this, and the source field will be vanishing soon.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>