mmc: npcm_sdhci: Fix use of CONFIG_IS_ENABLED on DM_REGULATOR

The DM_REGULATOR symbol has SPL counterpart in:
  drivers/power/regulator/Kconfig:config SPL_DM_REGULATOR
Use CONFIG_IS_ENABLED() macro to match on the correct
variant depending on the build stage.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/npcm_sdhci.c b/drivers/mmc/npcm_sdhci.c
index 7eb17cc..d63521d 100644
--- a/drivers/mmc/npcm_sdhci.c
+++ b/drivers/mmc/npcm_sdhci.c
@@ -36,7 +36,7 @@
 			return ret;
 	}
 
-	if (IS_ENABLED(CONFIG_DM_REGULATOR)) {
+	if (CONFIG_IS_ENABLED(DM_REGULATOR)) {
 		device_get_supply_regulator(dev, "vqmmc-supply", &vqmmc_supply);
 		vqmmc_uv = dev_read_u32_default(dev, "vqmmc-microvolt", 0);
 		/* Set IO voltage */