regulator: rt4801: Fix W=1 build warning when CONFIG_OF=n

Fix below warning when CONFIG_OF=n:

drivers/regulator/rt4801-regulator.c:206:34: warning: unused variable 'rt4801_of_id' [-Wunused-const-variable]
  206 | static const struct of_device_id rt4801_of_id[] = {
      |                                  ^~~~~~~~~~~~
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/1598234713-8532-1-git-send-email-u0084500@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/rt4801-regulator.c

@@ -203,7 +203,7 @@ static int rt4801_probe(struct i2c_client *i2c)
 	return 0;
 }
 
-static const struct of_device_id rt4801_of_id[] = {
+static const struct of_device_id __maybe_unused rt4801_of_id[] = {
 	{ .compatible = "richtek,rt4801", },
 	{ },
 };