PCI: imx6: Use gpio_set_value_cansleep()

We are in a context where we can sleep, and the PCIe reset gpio may be on
an I2C expander.  Use the cansleep() variant when setting the GPIO value.

Based on a patch from Russell King for pci-mvebu.c.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>