hwrng: st - Fix missing clk_disable_unprepare() on error in st_rng_probe()

Fix the missing clk_disable_unprepare() before return
from st_rng_probe() in the error handling case.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>