Bluetooth: check for already existent channel before create new one

Move this check to before the channel time creation simplifies the code
and avoid memory allocation if the channel already exist.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Acked-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>