[media] DocBook: some fixes for DVB FE open()

The changeset dc9ef7d1 change the open() ioctl documentation to
match the V4L2 open(). However, some cut-and-pasted stuff doesn't
match what actually happens at the DVB core.

So, fix the documentation entry to be more accurate with the DVB
frontend open() specifics.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>