powerpc/xics: Add missing of_node_put() in error path

The device node obtained with of_find_compatible_node() should be
released by calling of_node_put().  But it was not released when
of_get_property() failed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
[mpe: Invert the sense of the if so we only need one return path]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>