V4L/DVB (6564): Move check before lock

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
CC: Antti Palosaari <crope@iki.fi>
CC: Carl Lundqvist <comabug@gmail.com>
CC: Aapo Tahkola <aet@rasterburn.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>