fpga: fpga-mgr: xilinx-spi: fix error messages on -EPROBE_DEFER

The current code produces an error message on devm_gpiod_get() errors even
when the error is -EPROBE_DEFER, which should be silent.

This has been observed producing a significant amount of messages like:

    xlnx-slave-spi spi1.1: Failed to get PROGRAM_B gpio: -517

Fix and simplify code by using the dev_err_probe() helper function.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Fixes: dd2784c0 ("fpga manager: xilinx-spi: check INIT_B pin during write_init")
Fixes: 061c97d1 ("fpga manager: Add Xilinx slave serial SPI driver")
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>