ASoC: Intel: sof_sdw: Add NULL check in asoc_sdw_rt_dmic_rtd_init()

mic_name returned by devm_kasprintf() could be NULL.
Add a check for it.

Signed-off-by: Chenyuan Yang <chenyuan0y@gmail.com>
Fixes: bee2fe44 ("ASoC: Intel: sof_sdw: use generic rtd_init function for Realtek SDW DMICs")
Link: https://patch.msgid.link/20250415194134.292830-1-chenyuan0y@gmail.com


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