i2c-nomadik: Fixup clock handling

Make sure to clk_prepare as well as clk_enable.

Signed-off-by: Philippe Begnic <philippe.begnic@stericsson.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>