gpio: mpc8xxx: Don't overwrite default irq_set_type callback

The per-SoC devtype structures can contain their own callbacks that
overwrite mpc8xxx_gpio_devtype_default.

The clear intention is that mpc8xxx_irq_set_type is used in case the SoC
does not specify a more specific callback. But what happens is that if
the SoC doesn't specify one, its .irq_set_type is de-facto NULL, and
this overwrites mpc8xxx_irq_set_type to a no-op. This means that the
following SoCs are affected:

- fsl,mpc8572-gpio
- fsl,ls1028a-gpio
- fsl,ls1088a-gpio

On these boards, the irq_set_type does exactly nothing, and the GPIO
controller keeps its GPICR register in the hardware-default state. On
the LS1028A, that is ACTIVE_BOTH, which means 2 interrupts are raised
even if the IRQ client requests LEVEL_HIGH. Another implication is that
the IRQs are not checked (e.g. level-triggered interrupts are not
rejected, although they are not supported).

Fixes: 82e39b0d85 ("gpio: mpc8xxx: handle differences between incarnations at a single place")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20191115125551.31061-1-olteanv@gmail.com
Tested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-mpc8xxx.c

@@ -377,7 +377,8 @@ static int mpc8xxx_probe(struct platform_device *pdev)
 	 * It's assumed that only a single type of gpio controller is available
 	 * on the current machine, so overwriting global data is fine.
 	 */
-	mpc8xxx_irq_chip.irq_set_type = devtype->irq_set_type;
+	if (devtype->irq_set_type)
+		mpc8xxx_irq_chip.irq_set_type = devtype->irq_set_type;
 
 	if (devtype->gpio_dir_out)
 		gc->direction_output = devtype->gpio_dir_out;