hwrng: stm32 - put IP into RPM suspend on failure

In case of an irrecoverable failure, put the IP into RPM suspend
to avoid RPM imbalance. I did not trigger this case, but it seems
it should be done based on reading the code.

Fixes: b17bc6eb ("hwrng: stm32 - rework error handling in stm32_rng_read()")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>