configure: Disable host-bsd USB on FreeBSD

It hasn't built since FreeBSD 8.x, and is disabled by a patch in the
FreeBSD ports tree.  FreeBSD is migrating to QEMU's libusb support.
Signed-off-by: NEd Maste <emaste@freebsd.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

configure

@@ -553,7 +553,9 @@ esac
 
 if [ "$bsd" = "yes" ] ; then
   if [ "$darwin" != "yes" ] ; then
-    usb="bsd"
+    if [ "$targetos" != "FreeBSD" ]; then
+      usb="bsd"
+    fi
     bsd_user="yes"
   fi
 fi