PM / devfreq: exynos-ppmu: remove useless assignment

The error code is propagated to the caller, so there is no need to keep
it additionally in the unused variable.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>