build/arm: Arm compilation warning fix

The arm cross compiler is issuing warnings for signed/unsigned
comparisons for ntohs.  ntohs returns a unsigned int, while
the counting variables are signed.  Fixed to allow -Werror
to work properly

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/ospf6d/ospf6_message.c b/ospf6d/ospf6_message.c
index ecc96f7..e71b410 100644
--- a/ospf6d/ospf6_message.c
+++ b/ospf6d/ospf6_message.c
@@ -1630,7 +1630,7 @@
 ospf6_send (struct in6_addr *src, struct in6_addr *dst,
             struct ospf6_interface *oi, struct ospf6_header *oh)
 {
-  int len;
+  unsigned int len;
   char srcname[64], dstname[64];
   struct iovec iovector[2];
 
diff --git a/ospfd/ospf_interface.c b/ospfd/ospf_interface.c
index af4f0a6..f41c4f1 100644
--- a/ospfd/ospf_interface.c
+++ b/ospfd/ospf_interface.c
@@ -1011,7 +1011,7 @@
   struct ospf_interface *voi;
   struct listnode *node;
   struct vertex_parent *vp = NULL;
-  int i;
+  unsigned int i;
   struct router_lsa *rl;
 
   voi = vl_data->vl_oi;
diff --git a/ospfd/ospf_vty.c b/ospfd/ospf_vty.c
index e5e5631..c6f3b40 100644
--- a/ospfd/ospf_vty.c
+++ b/ospfd/ospf_vty.c
@@ -3694,7 +3694,8 @@
 show_ip_ospf_database_router_links (struct vty *vty,
                                     struct router_lsa *rl)
 {
-  int len, i, type;
+  int len, type;
+  unsigned int i;
 
   len = ntohs (rl->header.length) - 4;
   for (i = 0; i < ntohs (rl->links) && len > 0; len -= 12, i++)