Bluetooth: hci_uart: Fix init order in hci_uart bootup

bt_enable_raw() needs to be called before spawning the tx thread,
otherwise there might be an HCI command processed from the UART
before the HCI driver has been opened and therefore initialized.

Change-Id: I050158bd48bebaf8fa2cf6b11efb54b531f70079
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>