clk: samsung: exynos-audss: Use local variable for controller's device

Store pointer to the controller's device in local variable to avoid
extracting it from platform device in each call. This will also simplify
code in the future, when runtime PM support is added.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1503302703-13801-5-git-send-email-m.szyprowski@samsung.com