powerpc: ppc4xx: drop unused variable

commit 0d36fe65f58391712e11a6621075f373216e5f00
"powerpc: ppc4xx: use gpiochip data pointer"
made the mm_gc local variable in ppc4xx_gpio_set()
redundant, and when GCC treats warnings as errors this
happens:

arch/powerpc/sysdev/ppc4xx_gpio.c: In function 'ppc4xx_gpio_set':
arch/powerpc/sysdev/ppc4xx_gpio.c:93:26: error:
unused variable 'mm_gc' [-Werror=unused-variable]
struct of_mm_gpio_chip *mm_gc = to_of_mm_gpio_chip(gc);
^
cc1: all warnings being treated as errors

Reported-by: kbuild test robot <[e-mail]>
Cc: Anatolij Gustschin <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: [e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

arch/powerpc/sysdev/ppc4xx_gpio.c

···
 static void
 ppc4xx_gpio_set(struct gpio_chip *gc, unsigned int gpio, int val)
 {
-	struct of_mm_gpio_chip *mm_gc = to_of_mm_gpio_chip(gc);
 	struct ppc4xx_gpio_chip *chip = gpiochip_get_data(gc);
 	unsigned long flags;