The dvb_unregister_device() function tests whether its argument is NULL
and then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.
Signed-off-by: NMarkus Elfring <elfring@users.sourceforge.net>
Reviewed-by: NStefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>