PM / devfreq: exynos-bus: Reduce goto statements and remove unused headers

Improve code readability by changing the goto statements
as well as eliminating a few more goto statements (related to return
paths). Moreover, remove unused header file and adds a missing <linux/of.h>.

Signed-off-by: Artur Świgoń <a.swigon@samsung.com>
[cw00.choi: Edit patch title and description]
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>