Bluetooth: ISO: Fix using BT_SK_PA_SYNC to detect BIS sockets

BT_SK_PA_SYNC is only valid for Broadcast Sinks which means socket used
for Broadcast Sources wouldn't be able to use the likes of getpeername
to read out the sockaddr_iso_bc fields which may have been update (e.g.
bc_sid).

Fixes: 0a766a0a ("Bluetooth: ISO: Fix getpeername not returning sockaddr_iso_bc fields")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>