PM / devfreq: exynos-bus: Add error log when fail to get devfreq-event

Adding an error log makes it easier to trace the function's error path.
Because the error code may be rewritten on return, print error code here.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>