Fix signed/unsigned comparison warning

Trivial warning fix.

Change-Id: I93b905b34155214fc2d211a0d9319f06b67fa193
Author: Edwin Vane <edwin.vane@intel.com>
Reviewed-by: Ariel J Bernal <ariel.j.bernal@intel.com>

adb/adb.c

@@ -1067,7 +1067,7 @@ void connect_device(char* host, char* buffer, int buffer_size)
 
     strncpy(hostbuf, host, sizeof(hostbuf) - 1);
     if (portstr) {
-        if (portstr - host >= sizeof(hostbuf)) {
+        if ((unsigned int)(portstr - host) >= sizeof(hostbuf)) {
             snprintf(buffer, buffer_size, "bad host name %s", host);
             return;
         }