FPGA Manager fixes for 5.15

Tom and Jiapeng's fixes address smatch warnings around missing return
values in error cases.

Russ' change addresses an issue where registers are being accessed too
early resulting in invalid data being read.

All patches have been reviewed on the mailing list, and have been in the
last few linux-next releases (as part of my fixes branch) without issues.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
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