hwrng: stm32 - fix pm_suspend issue

When suspend is called after pm_runtime_suspend,
same callback is used and access to rng register is
freezing system. By calling the pm_runtime_force_suspend,
it first checks that runtime has been already done.

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