Bluetooth: ISO: Add check to prevent premature BIG termination

There was an issue where terminating the BIG while in the
ISO connected callbacks could lead to continueing the loops
would cause access to cleared memory.

The simple solution to this is to simply prevent
bt_iso_big_terminate from terminating the BIGs while we are
processing the BIG HCI events.

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