Bluetooth: ISO: Avoid creating child socket if PA sync is terminating

When a PA sync socket is closed, the associated hcon is also unlinked
and cleaned up. If there are no other hcons marked with the
HCI_CONN_PA_SYNC flag, HCI_OP_LE_PA_TERM_SYNC is sent to controller.

Between the time of the command and the moment PA sync is terminated
in controller, residual BIGInfo reports might continue to come.
This causes a new PA sync hcon to be added, and a new socket to be
notified to user space.

This commit fixs this by adding a flag on a Broadcast listening
socket to mark when the PA sync child has been closed.

This flag is checked when BIGInfo reports are indicated in
iso_connect_ind, to avoid recreating a hcon and socket if
residual reports arrive before PA sync is terminated.

Signed-off-by: Iulia Tanasescu <iulia.tanasescu@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>