ALSA: control: Introduce snd_ctl_find_id_mixer()

A commonly seen pattern is to run snd_ctl_find_id() for a mixer
control element with a given string.  Let's provide a standard helper
for achieving that for simplifying the code.

Link: https://lore.kernel.org/r/20230720082108.31346-2-tiwai@suse.de


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