Bluetooth: Free up l2cap_chan->sport when initiating a connection

The sport variable is used to track the allocation of the local PSM
database to ensure no two sockets take the same local PSM. It is
acquired upon bind() but needs to be freed up if the socket ends up
becoming a client one. This patch adds the clearing of the value when
l2cap_chan_connect is called.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>