bluetooth: Tune stack sizes after measuring with hci_uart

The following readings were obtained after running the peripheral and
central_hr apps in qemu combined with the controller (hci_uart) on nRF51
and nRF52:

Main Stack 380
Idle Stack: 68
ISR stack: 532
Controller RX Stack: 388
HCI TX Stack: 516

The numbers set in this change provide a safety margin from the ones
measured empirically.

Change-id: Ice7ad7f081502e0ea1accf856a7937c0bf0783b2
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>