[media] stv0900: remove unnecessary null pointer check

The address of a variable is impossible to be null, so we remove the check.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>