Add clk_disable_unprepare() in the error path in stm32_rng_init().

Fixes: 6b85a7e1 ("hwrng: stm32 - implement STM32MP13x support")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>