Kconfig: Hide the "Bluetooth Drivers" menu entry when BT_CONTROLLER

It used to be that there was a fairly empty "Bluetooth Drivers" menu
entry in the drivers menu. This entry was present even though there
was no drivers/bluetooth code being compiled in.

With this patch "Bluetooth Drivers" will no longer be present when
BT_CONTROLLER is enabled.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>