xen-netfront: do not cast grant table reference to signed short

xen-netfront: do not cast grant table reference to signed short

While grant reference is of type uint32_t, xen-netfront erroneously casts
it to signed short in BUG_ON().

This would lead to the xen domU panic during boot-up or migration when it
is attached with lots of paravirtual devices.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
OraBug: 25497392

(cherry picked from commit 87557efc27f6a50140fb20df06a917f368ce3c66)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>