[media] au0828: Make the s_reg and g_reg advanced debug calls work against the bridge

The g_reg and s_reg calls worked properly if acting on subdev registers
(such as the au8522), but didn't work against the au0828 itself.  Copy
the logic over from em28xx.

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