[media] cx25840: Add a flag to enable the CX23888 DIF to be enabled or not

The CX23888 (HVR1850 and other new cards) contain a DIF which is responsible
for demodulating the audio and video. It's built directly into the CX23888
PCIe bridge. It needs to be enabled and disabled on a per-card and per-input
basis. We pass this flag from the cx23885 core driver to the sub-device
when we need the cx25840 driver to enable/disable with the DIF correctly.

Signed-off-by: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>