Support bt_disable on Silabs chips

[Sylphe88]

Is your enhancement proposal related to a problem? Please describe. Silabs chips do not support bt_disable (at least on the Series 2 EFR32MG24 we're using), returning -ENOTSUP, seemingly related to some on-demand start component missing. We'd like to have better control over BLE, especially for power management purposes, so we'd need to turn off the stack for limited power consumption, and reenable it when we are out of low-power.

Describe the solution you'd like Simply support the BT_DISABLE HCI state!

Describe alternatives you've considered

• Further power optimization (further reading in progress) that do not involve stopping the BLE controller

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