[media] DaVinci-VPBE: Reduce the scope for a variable in vpbe_set_default_output()

* Move the definition for the variable "ret" into an if branch
  so that an extra initialisation can be avoided at the beginning
  by this refactoring.

* Return a success code as a constant at the end.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>