media: cx18: update *pos correctly in cx18_read_pos()

We should be updating *pos.  The current code is a no-op.

Fixes: 1c1e45d1 ("V4L/DVB (7786): cx18: new driver for the Conexant CX23418 MPEG encoder chip")

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>