ALSA: usb-audio: Drop bogus dB range in too low level

ALSA: usb-audio: Drop bogus dB range in too low level

commit 21cba9c5359dd9d1bffe355336cfec0b66d1ee52 upstream.

Some USB audio firmware seem to report broken dB values for the volume
controls, and this screws up applications like PulseAudio who blindly
trusts the given data.  For example, Edifier G2000 reports a PCM
volume from -128dB to -127dB, and this results in barely inaudible
sound.

This patch adds a sort of sanity check at parsing the dB values in
USB-audio driver and disables the dB reporting if the range looks
bogus.  Here, we assume -96dB as the bottom line of the max dB.

Note that, if one can figure out that proper dB range later, it can be
patched in the mixer maps.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=211929
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210227105737.3656-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>