samples: Bluetooth: hci_ipc: Fix Newton's Cradle, reduce RAM usage

The required ISO Tx buffers have to match the Read Buffer
Size values, otherwise the difference in the value cause a
similar amount of buffers to be stalled in the IPC driver.

Use reduced HEAP size to make room for increase in RAM
usage.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>