cpufreq: cpu-boost: Use policy max when calculating sync frequency

* This was changed to cpuinfo max in https://github.com/zwliew/ghost/commit/82dc34e893fd0e4c107d0da7612411c54ada6d34.
Due to changes like user underclocking or thermal throttling,
we need to use the current policy max.

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>