ASoC: fsl_rpmsg: Check -EPROBE_DEFER for getting clocks

The devm_clk_get() may return -EPROBE_DEFER, then clocks
will be assigned to NULL wrongly. As the clocks are
optional so we can use devm_clk_get_optional() instead of
devm_clk_get().

Fixes: b73d9e62 ("ASoC: fsl_rpmsg: Add CPU DAI driver for audio base on rpmsg")
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1629266614-6942-1-git-send-email-shengjiu.wang@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>