PCI: rcar: Fix a potential NULL pointer dereference

In case __get_free_pages() fails and returns NULL, fix the return
value to -ENOMEM and release resources to avoid dereferencing a
NULL pointer.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>