V4L/DVB (6005): Initialize filp->private_data only once in em28xx_v4l2_open

Some lines later filp->private_data is initialized to dev again.
Since there are some checks that might fail in the mean time
keep the later version.

Signed-off-by: Rolf Eike Beer <eike-kernel@sf-tec.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>