i2c: designware: Return error in ->probe() when clock ungating fails

In the designware platdrv ->probe() the error code received from
clk_prepare_enable() is wrongly ignored. Let's change that by bailing out
and propagate the error code.

After this change i2c_dw_plat_prepare_clk() is only used when CONFIG_PM is
set, thus move it within that #ifdef to avoid compiler warnings.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>