fpga: Simplify error path in fpga_add

Check !desc earlier to simplify code.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Reviewed-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>