drivers: bluetooth: stm32wb: Put HCI messages under CONFIG_BT_HCI_HOST

HCI message sending in this hci driver as these actions require hci_host
to be available and could be performed by the remote host if required.
Explicit this dependency using CONFIG_BT_HCI_HOST.
This is is required to be able to compile BT_HCI_RAW mode.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>