i2c: mpc: Use devm_clk_get_optional_enabled() to simplify code

devm_clk_get_optional() and clk_prepare_enable() can be replaced by the
helper function devm_clk_get_optional_enabled(). Let's simplify the code by
using devm_clk_get_optional_enabled() and avoid calling
clk_disable_unprepare().

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>