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

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>