Fixup switch statement in Cacher driver setup

[Alphix]

I'm not a PHP guru but it seems that the switch statement would always
just go with the default.

[Alphix]

See issue #1576

[Alphix]

Update: seems I was wrong about the switch statement being incorrect (see #1576).

I still think my alternative is much cleaner though, so I'll leave this PR open and let the maintainers decide whether they agree or not.

[RainLoop]

Hi @Alphix .
Unfortunately, your code isn't the same as mine. It doesn't work correctly for some reasons.
For example, in your case if fast_cache_driver = 'acp', but apc_store function doesn't exist, then $oDriver stay null. It's a bug, because my code supports failback to the FILES driver.