ANDROID: GKI: restore a part of "struct mmc_host"

In commit 17a17bf5 ("mmc: core: Fix hanging on I/O during system
suspend for removable cards") that came in through an LTS update, the
struct mmc_host structure lost a field as it was no longer needed.

But, of course, the ABI check notices this, so put the field back, even
if it is no longer being used, as the ABI should be preserved here.
This allows the issue to still be resolved and everything remains
working properly.

Bug: 161946584
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I4c6ab947a60d01779a26c7544334482748648c4c