Bluetooth: ISO: Fix invalid context error

This moves the hci_le_terminate_big_sync call from rx_work
to cmd_sync_work, to avoid calling sleeping function from
an invalid context.

Reported-by:  <syzbot+c715e1bd8dfbcb1ab176@syzkaller.appspotmail.com>
Fixes: a0bfde16 ("Bluetooth: ISO: Add support for connecting multiple BISes")
Signed-off-by: Iulia Tanasescu <iulia.tanasescu@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>