fix string replace.

configure files with this would fail:

options wl x=1

were being handled as " =1".
diff --git a/libkmod/libkmod-util.c b/libkmod/libkmod-util.c
index 2e58c91..9a68a3e 100644
--- a/libkmod/libkmod-util.c
+++ b/libkmod/libkmod-util.c
@@ -208,10 +208,9 @@
 char *strchr_replace(char *s, int c, char r)
 {
 	char *p;
-
-	for (p = s; p != NULL; p = strchr(p, c))
-		*p = r;
-
+	for (p = s; *p != '\0'; p++)
+		if (*p == c)
+			*p = r;
 	return s;
 }