V4L/DVB: em28xx: remove unneeded null checks

The "dev" variable is used as a list cursor in a list_for_each_entry()
loop and can never be null here so I removed the check.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>