mmc: sdhci: Fix vqmmc error setting

supply.vqmmc is used with the IS_ERR macro which means
the value must be valid or an error code.  NULL is
neither, so replace with ERR_PTR(-EINVAL).
Signed-off-by: NAdrian Hunter <adrian.hunter@intel.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/sdhci.c

@@ -3096,7 +3096,7 @@ int sdhci_add_host(struct sdhci_host *host)
 		if (ret) {
 			pr_warn("%s: Failed to enable vqmmc regulator: %d\n",
 				mmc_hostname(mmc), ret);
-			mmc->supply.vqmmc = NULL;
+			mmc->supply.vqmmc = ERR_PTR(-EINVAL);
 		}
 	}