fpga: altera_freeze_bridge: Constify ops

The ops are not changing, make them const.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Cc: Alan Tull <atull@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-fpga@vger.kernel.org
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>