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: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>