fpga: make xlnx_pr_decoupler_br_ops const

Make this const as it is only passed to a const argument of the function
fpga_bridge_register.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>