[media] DaVinci-VPBE: Return the success indication only as a constant in vpbe_set_mode()

* Return a success code without storing it in an intermediate variable.

* Delete the local variable "ret" which became unnecessary with
  this refactoring.

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>