ALSA: usb-audio: Use snprintf instead of sprintf in build_mixer_unit_ctl

Simplified code by removing redundant assignment of sprintf return value
and improved safety by replacing sprintf with snprintf.

Signed-off-by: Zhu Jun <zhujun2@cmss.chinamobile.com>
Link: https://patch.msgid.link/20241009070944.6937-1-zhujun2@cmss.chinamobile.com


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