[media] au8522: don't attempt to configure unsupported VBI slicer

Since we don't suppoort sliced VBI with the au0828/au8522, there is
no need to configure the au8522 VBI slicer, which because of the
coefficients requires a large amount of i2c traffic.

Remove the relevant code.  Note that this has no effect on raw VBI
support, which is currently the only supported way to access VBI on
this device.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>