powerpc: ppc4xx: drop unused variable

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 <fengguang.wu@intel.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>