fpga: machxo2-spi: Address warning about unused variable

Address warning about unused variable in case CONFIG_OF is not set.

warning: unused variable 'of_match' [-Wunused-const-variable]
static const struct of_device_id of_match[] = {

Fixes: 88fb3a002330 ("fpga: lattice machxo2: Add Lattice MachXO2 support")
Cc: Arnd Bergmann <[e-mail]>
Cc: Tom Rix <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Reported-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Moritz Fischer <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/fpga/machxo2-spi.c

···
 	return devm_fpga_mgr_register(dev, mgr);
 }
 
+#ifdef CONFIG_OF
 static const struct of_device_id of_match[] = {
 	{ .compatible = "lattice,machxo2-slave-spi", },
 	{}
 };
 MODULE_DEVICE_TABLE(of, of_match);
+#endif
 
 static const struct spi_device_id lattice_ids[] = {
 	{ "machxo2-slave-spi", 0 },