fpga: fpga-region: comment on fpga_region_program_fpga locking

Add a comment to the header of fpga_region_program_fpga()
regarding locking of the bridges.

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