alsa-utils: amixer: Fix SND_CTL_TLVT_DB_RANGE handling

Current support for handling SND_CTL_TLVT_DB_RANGE in amixer is completely
broken. This patch fixes it.

Reported-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>