V4L/DVB (10853): cx18: Fix s-parse warnings and a logic error about extracting the VBI PTS

My s-parse builds never griped about be32_to_cpu() casting to __be32, but Hans'
builds did.  This change explictly declares the pointer into the VBI buffer
header as __be32, which is the correct thing to do as the data is always
big endian when we go to fetch it.  Hopefully this will quiet s-parse warnings.
Also corrected a misplaced parenthesis logic error in checking for the VBI
header magic number.

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