gpio: mpc8xxx: Correct irq handler function

From the beginning of the gpio-mpc8xxx.c, the "handle_level_irq"
has being used to handle GPIO interrupts in the PowerPC/Layerscape
platforms. But actually, almost all PowerPC/Layerscape platforms
assert an interrupt request upon either a high-to-low change or
any change on the state of the signal.

So the "handle_level_irq" is not reasonable for PowerPC/Layerscape
GPIO interrupt, it should be "handle_edge_irq". Otherwise the system
may lost some interrupts from the PIN's state changes.

Signed-off-by: Liu Gang <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpio-mpc8xxx.c

···
 				irq_hw_number_t hwirq)
 {
 	irq_set_chip_data(irq, h->host_data);
-	irq_set_chip_and_handler(irq, &mpc8xxx_irq_chip, handle_level_irq);
+	irq_set_chip_and_handler(irq, &mpc8xxx_irq_chip, handle_edge_irq);
 
 	return 0;
 }