Unified Provisioning: Miscellaneous fixes in BLE

Unified Provisioning: Miscellaneous fixes in BLE

1. Pass the correct conn_id to protocomm_req_handle
In transport_simple_ble_write(), passing param->exec_write.conn_id would
be invalid. Instead param->write.conn_id should be passed
Similar change in transport_simple_ble_exec_write() to use
param->exec_write.conn_id

2. simple_ble_start() assumes that the mode is BLE only and enables
Bluetooth controller accordingly. For, cases having BT + BLE like Alexa
(Provisioning over BLE + Audio over classic BT), this assumption should
be removed.