media: exynos4-is: Change clk_disable to clk_disable_unprepare

The corresponding API for clk_prepare_enable is clk_disable_unprepare,
other than clk_disable.

Fix this by changing clk_disable to clk_disable_unprepare.

Fixes: b4155d7d

 ("[media] exynos4-is: Ensure fimc-is clocks are not enabled until properly configured")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>