Bluetooth: remove unnecessary check on BT_CLOSED socks during accept

BT_CLOSED sockets are unlinked from teardown callback. Trying to unlink
such sockets from bt_accept_dequeue may lead to NULL dereference as
teardown callback does not hold lock for parent sock.

bt_accept_unlink may be called from two threads, resulting in incorrect
socket ref count. Return if parent is NULL.

Change-Id: I957f0396079888549e082c34401322a6a8a80625
Signed-off-by: Rupesh Tatiya <rtatiya@codeaurora.org>