RDMA/uverbs: Unify return values of not supported command

The non-existing command is supposed to return -EOPNOTSUPP, but the
current code returns different errors for different flows for the
same failure. This patch unifies those flows.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>