PCI: xilinx: Check for __get_free_pages() failure

If __get_free_pages() fails, return -ENOMEM to avoid a NULL pointer
dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>