i2c: digicolor: use clk_disable_unprepare instead of clk_unprepare

since clk_prepare_enable() is used to get i2c->clk, we should
use clk_disable_unprepare() to release it for the error path.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>