drivers: entropy: ease runtime requirements on BT HCI

On platforms like nrf5340 there are 2 CPUs:
- one is the cpu_net which takes care of the radio stuff and
  owns the HW random generator
- one is the cpu_app which holds application data and polls
  cpu_net through HCI commands when it needs some random data.

The PSA core implemented in Mbed TLS needs random data at initialization
time, which happens early in the boot process. If we wait for BT to
be ready before issuing the HCI command, then PSA core intialization
will fail. In facts there is no need for the BT to be completely
initialized just to ask for some random data from the cpu_app to
the cpu_net since the HW random generator will likely be already
functional in the cpu_net.
So let's just try the HCI command and, if something is not right,
it will fail anyway. There's no need to anticipate the failure.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>