V4L/DVB (11092): cx18: Optimize processing of VBI buffers from the capture unit

Removed some unnecessary memcpy()'s by reworking the compress_*_vbi_buf()
functions.

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