IB: Avoid ib_modify_port() failure for RoCE devices

IB: Avoid ib_modify_port() failure for RoCE devices

IB CM calls ib_modify_port() irrespective of link layer. If the
failure is returned, the mad agent gets unregistered for those
devices. Recently, modify_port() hook was removed from some of the
low level drivers as it was always returning success. This breaks
rdma connection establishment over those devices.
For ethernet devices, Qkey violation and port capabilities are not
applicable. So returning success for RoCE when modify_port hook is
is not implemented.

Cc: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>