ASoC: qcom: lpass-cpu: Remove unnecessary clock checks

Clean up the clock calling code by removing numerous IS_ERR() checks by
just assigning the clock NULL; as this turn all used functions in the
clk API to nops.

Also include the word "optional" in the error message when failing to acquire
the optional osr clocks.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>