fpga: of-fpga-region: accept overlays that don't program FPGA

The FPGA may already have a static image programmed when
Linux boots.  In that case a DT overlay may be used to add
the devices that already exist.  This commit allows that
by shuffling the order of some checks.

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