staging: mt7621-pci: be sure gpio descriptor is null on fails

Function 'devm_gpiod_get_index_optional' returns NULL if the
descriptor is invalid and the error associated for the error
pointer is ENOENT. Sometimes if the pin is just assigned the
error associated for the pointer might not be ENOENT but other.
In order to avoid weirds behaviours if this happen set descriptor
to NULL in the driver port structure.
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20200320110123.9907-6-sergio.paracuellos@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-pci/pci-mt7621.c

@@ -363,8 +363,10 @@ static int mt7621_pcie_parse_port(struct mt7621_pcie *pcie,
 
 	port->gpio_rst = devm_gpiod_get_index_optional(dev, "reset", slot,
 						       GPIOD_OUT_LOW);
-	if (IS_ERR(port->gpio_rst))
+	if (IS_ERR(port->gpio_rst)) {
 		dev_notice(dev, "Failed to get GPIO for PCIe%d\n", slot);
+		port->gpio_rst = NULL;
+	}
 
 	port->slot = slot;
 	port->pcie = pcie;