powerpc/sysdev/simple_gpio: Fix oops in gpio save_regs function

of_mm_gpiochip_add_data() generates an oops for NULL pointer dereference.

of_mm_gpiochip_add_data() calls mm_gc->save_regs() before
setting the data, therefore ->save_regs() cannot use gpiochip_get_data()

Fixes: 937daafca774 ("powerpc: simple-gpio: use gpiochip data pointer")
Cc: stable@vger.kernel.org # v4.7+
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/sysdev/simple_gpio.c

@@ -75,7 +75,8 @@ static int u8_gpio_dir_out(struct gpio_chip *gc, unsigned int gpio, int val)
 
 static void u8_gpio_save_regs(struct of_mm_gpio_chip *mm_gc)
 {
-	struct u8_gpio_chip *u8_gc = gpiochip_get_data(&mm_gc->gc);
+	struct u8_gpio_chip *u8_gc =
+		container_of(mm_gc, struct u8_gpio_chip, mm_gc);
 
 	u8_gc->data = in_8(mm_gc->regs);
 }