bluetooth: controller: Only BT_CTLR_ECDH default y if BT_HCI_RAW

In https://github.com/zephyrproject-rtos/zephyr/pull/84268


the ability to use the controller for ECDH was removed from
the host.

This means that BT_CTLR_ECDH is now only useful when using
BT_HCI_RAW.

Signed-off-by: Sean Madigan <sean.madigan@nordicsemi.no>