ALSA: hdspm - Fix compile warnings with PPC

The char can be unsigned on some architectures.  Since the code checks
the negative values, they should be declared as signed char explicitly.

  sound/pci/rme9652/hdspm.c:5449: warning: comparison is always false due to limited range of data type
  sound/pci/rme9652/hdspm.c:5462: warning: comparison is always false due to limited range of data type
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/pci/rme9652/hdspm.c

@@ -896,11 +896,11 @@ struct hdspm {
 	unsigned char max_channels_in;
 	unsigned char max_channels_out;
 
-	char *channel_map_in;
-	char *channel_map_out;
+	signed char *channel_map_in;
+	signed char *channel_map_out;
 
-	char *channel_map_in_ss, *channel_map_in_ds, *channel_map_in_qs;
-	char *channel_map_out_ss, *channel_map_out_ds, *channel_map_out_qs;
+	signed char *channel_map_in_ss, *channel_map_in_ds, *channel_map_in_qs;
+	signed char *channel_map_out_ss, *channel_map_out_ds, *channel_map_out_qs;
 
 	char **port_names_in;
 	char **port_names_out;