Refactor direct NPU ops to use a common convention

Currently, the direct NPU ops have different return types and must be
called manually with different handling in each instance. To make the
ops easier to use, they now have a common return type and can be called
with the same API.

Change-Id: Ibf8283eb0b83451b8ba352853bf53cdec9490cda
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>