V4L/DVB (11109): au0828: Fix compilation when VIDEO_ADV_DEBUG = n

As reported by Kyle McMartin and Randy Dunlap:

drivers/media/video/au0828/au0828-video.c:1438: error: 'const struct v4l2_subdev_core_ops' has no member named 'g_register'
drivers/media/video/au0828/au0828-video.c:1453: error: 'const struct v4l2_subdev_core_ops' has no member named 's_register'

This patch properly implements those two API ioctls only when debug is
enabled.

Cc: Devin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>