ALSA: Add const prefix to proc helper functions

Add appropriate const prefix to char * arguments in proc helper functions.
Also fixed the caller side to be proper const pointers.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/oss/mixer_oss.c

@@ -1154,7 +1154,8 @@ static void snd_mixer_oss_proc_write(struct snd_info_entry *entry,
 				     struct snd_info_buffer *buffer)
 {
 	struct snd_mixer_oss *mixer = entry->private_data;
-	char line[128], str[32], idxstr[16], *cptr;
+	char line[128], str[32], idxstr[16];
+	const char *cptr;
 	int ch, idx;
 	struct snd_mixer_oss_assign_table *tbl;
 	struct slot *slot;

sound/core/oss/pcm_oss.c

@@ -2836,7 +2836,8 @@ static void snd_pcm_oss_proc_write(struct snd_info_entry *entry,
 				   struct snd_info_buffer *buffer)
 {
 	struct snd_pcm_str *pstr = entry->private_data;
-	char line[128], str[32], task_name[32], *ptr;
+	char line[128], str[32], task_name[32];
+	const char *ptr;
 	int idx1;
 	struct snd_pcm_oss_setup *setup, *setup1, template;