cpufreq: exynos: Remove unused variable & IS_ERR

The variable 'max_support_idx, min_support_idx, pm_lock_idx"
are never used, so remove the unused variable.

Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>