Bluetooth: host: downgrade select log messages

In the case of a constrained system that uses dynamic channels (ie, not
much initial credits), the user's log would be flooded with those three
messages.

This is not really an error per se as the stack will handle it properly and
reschedule the sending of the buffers as soon as more credits arrive.

Thus downgrading the severity of
- the l2cap messages, as they are only compiled when dynamic channels are
enabled
- the conn message. Reporting the error belongs in the upper layers.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>