clk: s2mps11: Use kcalloc instead of kzalloc for array allocation

This patch cleans up the driver for,

  - Use devm_kcalloc() variant instead of devm_kzalloc() for array
    allocation.
  - clk_prepare()/unprepare(), remove "ret" variable as it is not required
  - use __exit for cleanup function

As I am referring this driver as a reference for my 88pm800 clk driver,
applying same changes here as well.

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>