crypto: s5p-sss - remove redundant null check

Because clk_disable_unprepare already checked NULL clock
parameter, so the additional checks are unnecessary, just remove them.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Kamil Konieczny <k.konieczny@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>