i2c: piix4: Pre-shift the port number

Shift the port number at initialization time, so that it is ready to
use at run time. That way we don't have to do it again for every SMBus
transaction.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>