ath10k: Use standard bulk clock API in snoc

No frequency is currently specified for the single clock defined in the
snoc driver, so the clock wrappers reimplements the standard bulk API
provided by the clock framework. Change to this.

The single clock defined is marked as optional so this version of the
get API is used, but might need to be reconsidered in the future.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>