hwrng: atmel - use clk_disable_unprepare instead of clk_disable

Since clk_prepare_enable() is used to get trng->clk, we should
use clk_disable_unprepare() to release it for the error path.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>