PCI: mvebu: Remove unneeded gpiod NULL check

The gpiod API checks for NULL descriptors, so there is no need to duplicate
the check in the driver.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>