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: 88fb3a00 ("fpga: lattice machxo2: Add Lattice MachXO2 support")
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Tom Rix <trix@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Link: https://lore.kernel.org/r/20210618224618.1487323-1-mdf@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/fpga/machxo2-spi.c

@@ -374,11 +374,13 @@ static int machxo2_spi_probe(struct spi_device *spi)
 	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 },