Bluetooth: Audio: Unicast client missing unref on ACL disconnected

The unicast client takes a reference to the ACL, but did
not return the reference when the ACL disconnected, but
rather just reset everything.

Modified to detach the stream, ensuring
a bt_conn_unref on ACL disconnect.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>