net: qede: use return from qede_flow_spec_validate_unused()

When calling qede_flow_spec_validate_unused() then
the return code was only used for a non-zero check,
and then -EOPNOTSUPP was returned.

qede_flow_spec_validate_unused() can currently fail with:
* -EOPNOTSUPP

This patch changes qede_flow_spec_to_rule() to use the
actual return code from qede_flow_spec_validate_unused(),
so it's no longer assumed that all errors are -EOPNOTSUPP.

Only compile tested.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>