alsa-mixer: disable has_dB if max_dB is negative

Volume scaling in dB mode is broken if max dB is negative.

I have a Nobsound USB amplifier (1908:2220) that reports a dB range
of -127.07 dB to -128 dB in Alsa.
While this is likely a driver/device bug, in my naive imagination
userspace wouldn't bother too much with the absolute values and just set

	out_dB(percent) = min_dB + (max_dB - min_dB) * percent

However, this is not what PulseAudio is doing, instead max_dB is used
as base_volume with which the desired software volume is multiplied
while min_dB does not seem to be taken into account.

The result is that with this device only a tiny portion of the volume
slider is usable.
Setting it to 97% already reaches min_dB which effectively turns any
(software) audio knob to an on/off switch.

To work around this, simply set the has_dB flag to false if max_dB is
negative.
This falls back to using raw Alsa values (ranging from 0 - 255), now
the settings in pavucontrol perfectly mirror those in alsamixer.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/447>

src/modules/alsa/alsa-mixer.c

@@ -1711,6 +1711,14 @@ static bool element_probe_volume(pa_alsa_element *e, snd_mixer_elem_t *me) {
     else
         e->has_dB = snd_mixer_selem_get_capture_dB_range(me, &min_dB, &max_dB) >= 0;
 
+    /* Assume decibel data to be incorrect if max_dB is negative. */
+    if (e->has_dB && max_dB < 0) {
+        pa_alsa_mixer_id_to_string(buf, sizeof(buf), &e->alsa_id);
+        pa_log_warn("The decibel volume range for element %s (%li dB - %li dB) has negative maximum. "
+                    "Disabling the decibel range.", buf, min_dB, max_dB);
+        e->has_dB = false;
+    }
+
     /* Check that the kernel driver returns consistent limits with
      * both _get_*_dB_range() and _ask_*_vol_dB(). */
     if (e->has_dB && !e->db_fix) {