ASoC: cpcap: Check for NULL pointer after calling of_get_child_by_name

ASoC: cpcap: Check for NULL pointer after calling of_get_child_by_name

If the device does not exist, of_get_child_by_name() will return NULL
pointer.
And devm_snd_soc_register_component() does not check it.
Also, I have noticed that cpcap_codec_driver has not been used yet.
Therefore, it should be better to check it in order to avoid the future
dereference of the NULL pointer.

Fixes: f6cdf2d3445d ("ASoC: cpcap: new codec")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Link: https://lore.kernel.org/r/20220111025048.524134-1-jiasheng@iscas.ac.cn
Signed-off-by: Mark Brown <broonie@kernel.org>