hwrng: nomadik - Change clk_disable to clk_disable_unprepare

The corresponding API for clk_prepare_enable is clk_disable_unprepare,
other than clk_disable_unprepare.

Fix this by changing clk_disable to clk_disable_unprepare.

Fixes: beca35d0

 ("hwrng: nomadik - use clk_prepare_enable()")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>