Remove hard-coded values from widget-gain.c and widget-volume.c

Update alsa-sim.c and alsa.c to read the TLV info, and update the gain
and volume widgets to use that info rather than hard-coding the
min/max values.

src/alsa.h

@@ -115,6 +115,12 @@ struct alsa_elem {
   int         type;
   int         count;
 
+  // for gain/volume elements, the dB range and step
+  int min_val;
+  int max_val;
+  int min_dB;
+  int max_dB;
+
   // for the number (or translated letter; A = 1) in the item name
   // TODO: move this to struct routing_snk?
   int lr_num;