ethdev: expose flow API error helper

ethdev: expose flow API error helper

rte_flow_error_set() is a convenient helper to initialize error objects.

Since there is no fundamental reason to prevent applications from using it,
expose it through the public interface after modifying its return value
from positive to negative. This is done for consistency with the rest of
the public interface.

Documentation is updated accordingly.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>