mmc: core: use cache flush timeout as cache disable timeout

When eMMC cache is disabled, the card has to flush all the data in its
cache.
This change increases the timeout for cache flush operation and also uses
the cache flush timeout in cache disable operation.

Change-Id: I373050a99816c95e784a957de4271955a511b9f9
CRs-Fixed: 723701
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>