V4L/DVB (10856): cx18: Add interlock so sliced VBI insertion only happens for an MPEG PS

The cx18 private packet insertion code only expects to operate on an MPEG PS
when inserting private packets of sliced VBI data.  This patch keeps the cx18
driver from corrupting the MPEG TS or other non-PS stream types, in case the
user enables sliced VBI insertion for these MPEG stream types.

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