Bluetooth: host: Introduce a new bt_hci_le_rand() call

In order to get rid of the duplication of the code that we had until now
in the tree, consolidate the handling of multiple calls to
bt_hci_cmd_send_sync(BT_HCI_OP_LE_RAND, ...) in a single location,
namely in hci_core.

This allows all of the users of this HCI command to use a single
implementation of the iterated sending of the HCI command to fill a
buffer with random bytes.

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