i2c: stu300: use clk_prepare/unprepare

Make sure we prepare/unprepare the clock for the ST U300
I2C driver as is required by the clk API especially if you
use common clock.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>