mmc: sdhci: fix NULL dereference in sdhci_request() tuning

mmc: sdhci: fix NULL dereference in sdhci_request() tuning

commit 14efd957209461bbdf285bf0d67e931955d04a4c upstream.

Commit 473b095a72a9 ("mmc: sdhci: fix incorrect command used in tuning")
introduced a NULL dereference at resume-time if an SD 3.0 host controller
raises the SDHCI_NEEDS_TUNING flag while no card is inserted.  Seen on an
OLPC XO-4 with sdhci-pxav3, but presumably affects other controllers too.

Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>