HAL_RNG_GetRandomNumber is a deprecated API and replaced here with
a call to HAL_RNG_GenerateRandomNumber.
Doing so, we also rework the driver to use the 4 bytes returned by a call
to HAL_RNG_GenerateRandomNumber instead of 1 byte out of 4.
HAL_RNG_GenerateRandomNumber was not returning any error code, so now
we can also check the return code.