hwrng: stm32 - rework read timeout calculation

Increase timeout delay to support longer timing linked
to rng initialization. Measurement is based on timer instead
of instructions per iteration which is not powerful on all
targets.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>