fpga: do not access region struct after fpga_region_unregister

A couple drivers were accessing the region struct after it had been
freed.  Save off the pointer to the mgr before the region struct gets
freed.

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