media: ivtv: update *pos correctly in ivtv_read_pos()

We had intended to update *pos, but the current code is a no-op.

Fixes: 1a0adaf3 ("V4L/DVB (5345): ivtv driver for Conexant cx23416/cx23415 MPEG encoder/decoder")

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>