ALSA: ctxfi: Remove unnecessary cast in kfree

Remove unnecassary casts in the argument to kfree.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Link: https://lore.kernel.org/r/20200409112052.13402-1-vulab@iscas.ac.cn


Signed-off-by: Takashi Iwai <tiwai@suse.de>