ALSA: synth: Save a few bytes of memory when registering a 'snd_emux'

snd_emux_register() calls pass a string literal as the 'name' parameter.

So kstrdup_const() can be used instead of kfree() to avoid a memory
allocation in such cases.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/9e7b94c852a25ed4be5382e5e48a7dd77e8d4d1a.1705743706.git.christophe.jaillet@wanadoo.fr


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