V4L/DVB (10440): cx18: Fix presentation timestamp (PTS) for VBI buffers

The old code from ivtv used a CX23415/6 PTS, which was simply left at 0 in the
cx18 driver.  Since the CX23418 gives us what I think is a PTS (or some other
90 kHz clock count) with each VBI buffer, this change has the cx18 driver use
that as a PTS.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>