fix typo: libisb -> libusb

Change-Id: I0a9505e3eeb7ae4ffda7081dcbb1ed63835600a5
diff --git a/host/libusb_util.c b/host/libusb_util.c
index 45e3f50..71fb488 100644
--- a/host/libusb_util.c
+++ b/host/libusb_util.c
@@ -1,4 +1,4 @@
-/* libisb utilities
+/* libusb utilities
  * 
  * (C) 2010-2016 by Harald Welte <hwelte@hmw-consulting.de>
  *