snd_tlv_convert_to_dB: Fix mute handling for MINMAX_MUTE type

snd_tlv_convert_to_dB: Fix mute handling for MINMAX_MUTE type

Ensure the SND_CTL_TLV_DB_GAIN_MUTE value is returned when the calculated
gain equals the minimum dB value for the SNDRV_CTL_TLVT_DB_MINMAX_MUTE type.
The previous check based solely on the volume value could miss cases where
the linear calculation resulted in the minimum gain.

Closes: https://github.com/alsa-project/alsa-lib/pull/478
Signed-off-by: qaqland <anguoli@uniontech.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>