Bluetooth: Controller: Implement LE_RAND command

The LE_RAND HCI command was not filling in the rand array
with 8 random numbers as required by the spec.
Please note that the while() loop inside the command's
implementation can take up to hundreds of ms to execute.

Jira: ZEP-726

Change-Id: If27ff861ee5fa7842cd469e99d5bfa8ac47ac2fa
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>