ASoC: cht_bsw_rt5645: Fix leftover kmalloc

cht_bsw_rt5645 driver allocates the own codec_id string but doesn't
release it.  For simplicity, put the string in cht_mc_private; then
the string is allocated in a shot and released altogether.

Fixes: c8560b7c ("ASoC: cht_bsw_rt5645: Fix writing to string literal")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>