fix(sleep): Make sleep work again when BLE is disabled

In commit 4944aa48 ("fix(ble): Update to changes from new SDK")
a call to `BbDrvDisable()` was added to `sleep_deepsleep()`.  This
function must, however, only be called when BLE was previously
initialized, otherwise a wakeup from deepsleep will not be possible (if
it ever reaches it?).

Fix this by reworking the BLE enabled check to also be usable here, to
only call `BbDrvDisable()` when BLE is active.

Fixes: 4944aa48 ("fix(ble): Update to changes from new SDK")
Fixes: #231
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