fpga: xiilnx-spi: Address warning about unused variable

warning: ‘xlnx_spi_of_match’ defined but not used
 [-Wunused-const-variable]
  static const struct of_device_id xlnx_spi_of_match[] = {

Fixes: 061c97d1 ("fpga manager: Add Xilinx slave serial SPI driver")
Cc: Tom Rix <trix@redhat.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>