V4L/DVB: VIDEO: ivtvfb, remove unneeded NULL test

Stanse found that in ivtvfb_callback_cleanup and ivtvfb_callback_init
there are unneeded tests for itv being NULL. But itv is initialized
as container_of with non-zero offset in those functions, so it is
never NULL (even if v4l2_dev is). This was found because itv is
dereferenced earlier than the test.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>