Replace an open code with the new snd_ctl_find_id_mixer().
There is no functional change.

Also, add the missing NULL checks in psc724_set_jack_state() to deal
with error cases.

Link: https://lore.kernel.org/r/20230720082108.31346-7-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>