Win32 UI: Fix frameskipping menu & minor cleanup.

Picking frameskipping level 9 would never work due to how the loop was configured(it was set to check 0 to 8 instead of the full 0 to 9 range). Also clean up by removing magic numbers, instead using ARRAY_SIZE to loop. Fixes another issue with https://github.com/hrydgard/ppsspp/pull/2934.