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

commit 6f553912eedafae13ff20b322a65e471fe7f5236 upstream.

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: 937daafc ("powerpc: simple-gpio: use gpiochip data pointer")
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>