V4L/DVB (8879): bttv: Don't unmask VPRES interrupt

When the input is set to tuner and no antenna is connected, the BT848
can flood VPRES interrupts. So we don't want to enable this type of
interrupts when the input it set to tuner.

As we don't do anything when receiving such an interrupt anyway, the
easiest fix is to simply not unmask this specific interrupt.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>