cxl: replace loop with for_each_child_of_node(), remove unneeded of_node_put()

Rewrite the cxl_guest_init_afu() loop in cxl_of_probe() to use
for_each_child_of_node() rather than a hand-coded for loop.

Remove the useless of_node_put(afu_np) call after the loop, where it's
guaranteed that afu_np == NULL.

Reported-by: SF Markus Elfring <elfring@users.sourceforge.net>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>