mmc: sdhci: Fix potential null pointer access while accessing vqmmc

Don't access vqmmc regulator handler, if it's already invalidated.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>
Link: https://lore.kernel.org/r/1594300408-17658-1-git-send-email-vbadigan@codeaurora.org


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>