ALSA: usb-audio: Add minimal-mute notion in dB mapping table

Some devices do mute the volume at the minimal volume, and for such
devices, we need to set SNDRV_CTL_TLVT_DB_MINMAX_MUTE to the TLV
information.  It corresponds to setting usb_mixer_elem_info.min_mute
flag in the USB-audio driver.

This patch adds a new field min_mute in usbmix_dB_map so that the
mixer map entry can pass the flag.

Link: https://lore.kernel.org/r/20211116065415.11159-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.c

@@ -145,6 +145,7 @@ static inline void check_mapped_dB(const struct usbmix_name_map *p,
 	if (p && p->dB) {
 		cval->dBmin = p->dB->min;
 		cval->dBmax = p->dB->max;
+		cval->min_mute = p->dB->min_mute;
 		cval->initialized = 1;
 	}
 }

sound/usb/mixer_maps.c

@@ -8,6 +8,7 @@
 struct usbmix_dB_map {
 	int min;
 	int max;
+	bool min_mute;
 };
 
 struct usbmix_name_map {