ASoC: qcom: Modify test for DSP in LPASS driver

As the representation of the DSP in the device
tree has changed from a required subnode to an
optional phandle, modify the test for DSP
existence in the LPASS CPU DAI driver,
accordingly.

Signed-off-by: Kenneth Westfield <kwestfie@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>