drm/nouveau: remove unused variable from nouveau_bios_shadow_of

drivers/gpu/drm/nouveau/core/subdev/bios/base.c: warning: unused variable 'i'

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>