Bluetooth: use l2cap_chan_ready() instead of duplicate code

In this case the replacement by l2cap_chan_ready() doesn't change the code
flow, the same operations will executed plus two others that have no
effect: the use of the parent socket, that a non-oriented channel doesn't
have and the reset of conf_state, which is also fine since the connection
is ready at this point.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>