app_confbridge: Don't emit warnings on valid configurations.

app_confbridge: Don't emit warnings on valid configurations.

The numeric bridge profile options `internal_sample_rate` and
`maximum_sample_rate` are documented to accept the special values
`auto` and `none`, respectively. While these values currently work,
they also emit warnings when used which could be confusing for users.

In passing, also ensure that we only accept the documented range of
sample rate values between 8000 and 192000.

Fixes #546