Fix warnings that would be caused by gcc flag -Wwrite-strings

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5206 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/hw/usb-ohci.c b/hw/usb-ohci.c
index afd5506..55cb77b 100644
--- a/hw/usb-ohci.c
+++ b/hw/usb-ohci.c
@@ -565,7 +565,7 @@
 {
     int dir;
     size_t len = 0;
-    char *str = NULL;
+    const char *str = NULL;
     int pid;
     int ret;
     int i;
@@ -800,7 +800,7 @@
 {
     int dir;
     size_t len = 0;
-    char *str = NULL;
+    const char *str = NULL;
     int pid;
     int ret;
     int i;