xen-netfront: cast grant table reference first to type int

IS_ERR_VALUE() in commit 87557efc
("xen-netfront: do not cast grant table reference to signed short") would
not return true for error code unless we cast ref first to type int.

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