The driver-configurations are only set when creating a new controller,
using the `config.OptionDriverConfig()` option that can be passed to
`New()`, and used as "read-only" after that.
Taking away any other paths that set these options, the only type used
for per-driver options are a `map[string]interface{}`, so we can change
the type from `map[string]interface{}` to a `map[string]map[string]interface{}`,
(or its "modern" variant: `map[string]map[string]any`), so that it's
no longer needed to cast the type before use.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
738b16d873