PCI: imx6: Probe in module_init(), not fs_initcall()

This effectively reverts f216f57f ("PCI: imx6: Probe the PCIe in
fs_initcall()") as the resource allocation issue that prevented the driver
from working properly at module_initcall level is now fixed in
pcie-designware.c.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Richard Zhu <r65037@freescale.com>
(cherry picked from commit 61da50da)