Limit calls to free_rdma_netdev() for capable devices only.

Fixes: cd565b4b ('IB/IPoIB: Support acceleration options callbacks')
Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>