i2c: Fix Kontron PLD prescaler calculation

Add some necessary braces that have been removed during driver cleanup.
This fixes the I2C prescaler calculation.

Signed-off-by: Michael Brunner <michael.brunner@kontron.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>