drivers: entropy: stm32 rng driver should not block if Seed error occurs

When a Seed error occurs during the random nb generation,
the driver tries to recover and exit without providing a random data
This avoids looping endlessly on the DRDY bit of the RNG status reg
because it remains 0 in case of error.

Signed-off-by: Francois Ramu <francois.ramu@st.com>