ALSA: mixer_oss: Remove some incorrect kfree_const() usages

ALSA: mixer_oss: Remove some incorrect kfree_const() usages

"assigned" and "assigned->name" are allocated in snd_mixer_oss_proc_write()
using kmalloc() and kstrdup(), so there is no point in using kfree_const()
to free these resources.

Switch to the more standard kfree() to free these resources.

This could avoid a memory leak.

Fixes: 454f5ec1d2b7 ("ALSA: mixer: oss: Constify snd_mixer_oss_assign_table definition")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/63ac20f64234b7c9ea87a7fa9baf41e8255852f7.1727374631.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Takashi Iwai <tiwai@suse.de>