staging: mmal-vchiq: If the VPU returns an error, don't negate it

staging: mmal-vchiq: If the VPU returns an error, don't negate it

There is an enum for the errors that the VPU can return.
port_parameter_get was negating that value, but also using -EINVAL
from the Linux error codes.
Pass the VPU error code as positive values. Should the function
need to pass a Linux failure, then return that as negative.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>