i2c: stu300: do not request a specific clock name

We have used the default clock associated with the block
for a long time, only heuristics in the clock system has
made this work anyway. This needs to be done away with as
we start probing this driver and its clocks exclusively
from the device tree.

Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>