drm/xen-front: Fix loop timeout

If the loop times out then we want to exit with "to" set to zero, but in
the current code it's set to -1.

Fixes: c575b7ee

 ("drm/xen-front: Add support for Xen PV display frontend")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508092829.GC661@mwanda